He prays in the mosque with the congregation but he is always on his own behind the last row

Praying in congregation
is one of the greatest outward symbols of our faith. We have already stated
that the most correct view, based on the evidence, is that it is obligatory.
See question no.
120 

With regard to the prayer of one who prays alone behind the
last row, if there is space for him in the row, the scholars differed as to
whether his prayer is valid. Imam al-Tirmidhi (may Allaah have mercy on him)
said: Some of the scholars regarded it as invalid for a man to pray on his
own behind the row. They said that he should repeat the prayer if he prayed
behind the row on his own. This was also the view of Ahmad and Ishaaq. 

He also narrated this view from Hammaad ibn Abi Sulaymaan,
Ibn Abi Layla and Wakee’. 

And he said: Some of the scholars said that his prayer is
good enough if he prays behind the row on his own. This is the view of
Sufyaan al-Thawri, Ibn al-Mubaarak and al-Shaafa’i. 

The more correct view, which is that of Imam Ahmad and other
scholars, is that it is not permissible for a person to pray on his own
behind the row if it is possible for him to stand in the row; if he does
that (i.e., stands on his own behind the row) then his prayer is invalid and
he has to repeat it. 

This is indicated by the hadeeth narrated by Abu Dawood
(682), al-Tirmidhi (230) and others, according to which a man prayed on his
own behind the row and the Messenger of Allaah (peace and blessings of
Allaah be upon him) commanded him to repeat the prayer. Classed as saheeh by
al-Albaani in al-Irwa’. 

Al-Mubaarakfoori (may
Allaah have mercy on him) said: This indicates that prayer offered standing
on one’s own behind the row is not valid, and that whoever prays on his own
behind the row has to repeat his prayer. End quote. 

That is also indicated by the hadeeth narrated by Ahmad
(15862) and Ibn Maajah (1003), according to which ‘Ali ibn Shaybaan came in
a delegation to the Messenger of Allaah (peace and blessings of Allaah
be upon him). He said: we prayed behind the Prophet (peace and
blessings of Allaah be upon him), and he saw out of the corner of his eye a
man who did not make his back straight when he bowed and prostrated. When
the Messenger of Allaah (peace and blessings of Allaah be upon him) had
finished praying, he said: “O Muslims, there is no prayer for one who does
not make his back straight when he bows and prostrates.” And he saw a man
praying behind the row, so he waited until the man had finished, then the
Messenger of Allaah (peace and blessings of Allaah be upon him) said to
him: “Repeat your prayer, for there is no prayer for a man who stands alone
behind the row.” Classed as saheeh by Saheeh Ibn Maajah. 

Al-Sindi (may Allaah have mercy on him) said in his
commentary on Ibn Maajah: The apparent meaning of this hadeeth is that the
prayer of one who does that is invalid.  

Al-San’aani (may Allaah have mercy on him) said: 

This indicates that the prayer of one who prays behind the
row on his own is invalid, because this is the apparent meaning of the
hadeeth. 

Al-Nakha’i and Ahmad were also of the view that such a prayer
is invalid. Al-Shaafa’i regarded this hadeeth as da’eef (weak) and said: If
this hadeeth were proven to be sound I would follow it. Al-Bayhaqi said: The
best idea is to avoid doing that because this report is proven to be sound.
End quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was
asked: what is the ruling on praying behind the row on one’s own?  

He said: It is not permissible to pray behind the row on
one’s own and that prayer is not valid, according to the most sound
scholarly view. This is the well known view of Ahmad (may Allaah have mercy
on him), although there is another view reported from him which says that it
is valid. The view that the prayer of a person who stands on his own behind
the row is valid is the view of the other three imams, Maalik, Abu Haneefah
and al-Shaafa’i. 

But the more correct view
is that it is not valid to pray behind the row on one’s own, unless there is
no room to stand in the row. If the row is full and a person prays behind
the row standing on his own and following the imam, then he is excused, and
one is not obliged to do something that one is unable to do, as was stated
by the scholars (may Allaah have mercy on them). 

Fataawa al-Shaykh, 15/193 

In order for the one who does this action that has been asked
about to understand the seriousness of it, you should know that some of the
scholars are the view that the prayer of the one who prays on his own behind
the row is invalid in all cases, even if he cannot find a space in the row.
It says in Fataawa al-Lajnah al-Daa’imah: If a man enters the mosque
and the iqaamah for prayer has been given and the row is full, he should try
to enter the row. If he cannot do that then he should join the imam and
stand on his right. If he cannot do that then he should wait for someone
else to come with whom he can form a new row. If no one else comes then he
should pray on his own after the congregational prayer finishes.  

The same details were
mentioned in another answer given by the Committee, in which it says that it
is not sufficient to stand next to boys if they are under the age of
discernment. They said: with regard to standing next to boys, if they have
reached the age of discernment then standing next to them is valid, but if
they have not reached the age of discernment then the ruling is the same as
that on one who stands on his own behind the row, and the prayer of one who
stands on his own behind the row is not valid, because the Prophet
(peace and blessings of Allaah be upon him) said: “There is no prayer for
one who stands on his own behind the row.” 

See Fataawa al-Lajnah, 8/6-7 

Undoubtedly the situation which you have asked about goes
completely against the wisdom behind prayer in congregation. Anyone who has
the slightest understanding of the aims and purposes of sharee’ah would
reject this. Shaykh al-Islam Ibn Taymiyah (may Allaah have mercy on him)
said, after quoting the two ahaadeeth which are stated as evidence that the
prayer of one who prays on his own behind the row is invalid: 

More than one of the scholars of hadeeth have classed these
two hadeeths as saheeh, and their isnaads are strong. Indeed, in many cases
those who disagree with them rely on reports that have weaker isnaads than
these two. There is nothing in these two hadeeths that goes against the
basic principles of Islam, rather what is in them is what is implied by the
well known texts and the basic principles, because prayer in congregation is
called jamaa’ah because it brings together (ijtimaa’) the worshippers in
doing something at the same time and in the same place. If they cancel out
that unity of time or place, such as if all or some of them were to stand in
front of the imam, or to keep far back from him with no excuse, that is
forbidden according to scholarly consensus. Similarly if they were to be
scattered instead of standing together, this is something which is most
emphatically forbidden. Rather they are commanded to stand firmly and
closely together, and the Prophet (peace and blessings of Allaah be
upon him) commanded them to make the rows compact, straight and close
together, and to fill all the gaps, starting from the front row. This is all
aimed at joining them together in the best way possible. 

If forming rows was not obligatory, then it would be
permissible for one person to stand behind another and so on. Everyone knows
that this is not the way that Muslims pray. If that were permissible then
the Muslims would have done it, even if only once. The same would apply if
they made the rows uneven, with one standing ahead of another and one
standing back from another. But that is something that the Prophet
(peace and blessings of Allaah be upon him) forbade. 

If they pray in front of the imam, this is better than
praying without organized rows. If the majority of scholars say that praying
ahead of the imam is not correct, either in all cases or if there is no
excuse, how could prayer be valid without having properly organized rows? 

Study of the original sources shows that it is obligatory to
pray in rows, and that the prayer of one who stands alone is not valid, as
it says in these two hadeeths. If any scholar disagreed with that, then
undoubtedly this Sunnah did not reach him in a trustworthy report; he may
not have heard it at all, or he may have thought that the hadeeth was da’eef
(weak), as some of them said. 

Majmoo’ Fataawa Shaykh al-Islam,
23/393-395 

Try to offer advice to
this brother who is doing this, and tell him of the Sunnah of the Prophet
(peace and blessings of Allaah be upon him), and the fact that he
forbade doing such things. But your advice must be given in a kind and
gentle manner, choosing the circumstances in which you hope that your advice
will be accepted. May Allaah help us and you to do that which He loves and
which pleases him.