It is not permissible to make accusations without proof against a Muslim woman who does not wear hijab, or to pray that she go to Hell

Praise
be to Allah.

Firstly: 

Undoubtedly not wearing hijab is a major sin, but it does not make it
permissible for anyone to state categorically that the one who does not wear
hijab is one of the people of Hell. Likewise, it is not permissible to pray
against her in such terms, let alone indulge in impugning her honour and
speaking ill of her. Rather what is prescribed in Islam is to pray for her
that she be guided and her affairs set straight. 

Al-Bukhaari (6443) and Muslim (94) narrated from Abu Dharr (may Allah be
pleased with him) from the Prophet (blessings and peace of Allah be upon
him) that Jibreel (peace be upon him) said to him: “ ‘Give your ummah the
glad tidings that whoever dies not associating anything with Allah will
enter Paradise.’ I said: ‘O Jibreel, even if he steals and commits zina?’ He
said: ‘Yes.’ I said: ‘Even if he steals and commits zina?’ He said: ‘Yes.’ I
said: ‘Even if he steals and commits zina?’ He said: ‘Yes, and even if he
drinks alcohol.’” 

The scholars of the Standing Committee said: 

The belief of Ahl as-Sunnah wa’l-Jamaa‘ah is that any Muslim who dies
persisting in a major sin such as zina, slander or stealing is subject to
the will of Allah, may He be glorified: if Allah wills, He will forgive him,
and if Allah wills, He will punish him for the major sin that he was
persisting in when he died, but his ultimate destination will be Paradise,
because Allah, may He be glorified and exalted, says (interpretation of the
meaning): “Verily,
Allah forgives not that partners should be set up with him in worship, but
He forgives except that (anything else) to whom He pleases”
[an-Nisa’ 4:48]. And
there are mutawaatir hadiths which indicate that the sinners among those who
affirmed the Oneness of Allah will be brought forth from the Fire.

End quote from Fataawa al-Lajnah ad-Daa’imah (1/728) 

Please see also the answer to question no. 7436 

Abu Dawood (4906) narrated from Samurah ibn Jundub that the Prophet
(blessings and peace of Allah be upon him) said: “Do not invoke the curse of
Allah upon one another, or the wrath of Allah, or Hell.”

Classed as hasan by al-Albaani. 

It
says in ‘Awn al-Ma‘bood: 

“Do not invoke the curse of Allah upon one another” means: do not curse one
another, so no one should say to a particular Muslim: May the curse of Allah
be upon you, for example.

“or the wrath of Allah”, by saying: May the wrath of Allah be upon you

“or Hell” by saying: May Allah admit you to the Fire, for example. It
specifically mentions the prohibition on invoking such things upon
particular individuals, because it is permissible to invoke curses in
general terms, such as saying, May the curse of Allah be upon the
disbelievers, or to invoke curses upon specific groups of people, or a
specific disbeliever who died in a state of disbelief, such as Pharaoh or
Abu Jahl. End quote. 

Please see also the answer to question no. 83390 and
36674

Thirdly: 

It
is not permissible for a Muslim to accuse any Muslim man or woman of zina
without proof as specified in sharee‘ah, namely four witnesses of good
character, or a confession, or pregnancy in a woman who does not have a
husband. Whoever does that has committed a major sin and deserves the hadd
punishment for slander, which is eighty lashes on his back, and his
testimony is never to be accepted in the future, and he is not to be
described as being of good character, rather he is to be labelled as a
faasiq (evildoer). 

Please see the answers to questions no. 104486 and
121059 

With regard to them saying that the “chaste women, who never even think of
anything touching their chastity” are the women who stay in their husbands’
houses and wear hijab as prescribed in Islam, it may be noted that the verse
in which Allah, may He be exalted, says (interpretation of the meaning):
“Verily,
those who accuse chaste women, who never even think of anything touching
their chastity and are good believers, are cursed in this life and in the
Hereafter, and for them will be a great torment”
[an-Noor 24:23] refers
to every chaste Muslim woman who refrains from zina and is not known to
commit it. At-Tabari (may Allah have mercy on him) said: 

The “chaste women, who
never even think of anything touching their chastity”
are those who refrain from shameful deeds

“and
are good believers” in
Allah and His Messenger. 

End quote from Tafseer at-Tabari (19/138) 

It
is not permissible to slander any Muslim woman who is not proven to have
committed shameful deeds, even if the woman who is accused does not wear
hijab. 

It
says in al-Mawsoo‘ah al-Fiqhiyyah (2/227): 

The chaste individual (man or woman) whose accuser is to be subjected to the
hadd punishment is the one who fulfils the following conditions, according
to the consensus of the fuqaha’,if the accusation against him or her is of
zina: 

1.    
He or she should
be free; there is no hadd punishment for the one who makes accusations
against a slave, male or female.

2.    
He or she should
be Muslim; there is no hadd punishment for one who makes an accusation
against an apostate or one who was originally a disbeliever.

3.    
He or she should
be of sound mind.

4.    
He or she should
be an adult.

5.    
He or she should
be chaste. What is meant by being chaste is that the individual who is
accused never had unlawful sexual intercourse, or intercourse within the
framework of a marriage contract that is invalid according to scholarly
consensus. If he or she was ever involved in any such deed, the description
of chastity no longer applies.

 So chastity, in a practical sense, is a condition according to three (of
the four) imams. 

However, Imam Ahmad said:

An
individual may be described as chaste if he appears to refrain from zina. So
if a person has never been proven to have committed zina, on the basis of
evidence or a confession, and has never received a hadd punishment for zina,
he is to be regarded as chaste. 

As
there is no proof that this woman committed the crime of zina, she is
regarded as chaste according to the rules of sharee‘ah, and the one who
accused her without proof deserves to be subjected to the hadd punishment.
The mere fact that she does not wear hijab does not cancel out the
description of her as chaste. 

So
how about if, in addition to that, she is as you described, keen to pray
regularly and do good deeds, and memorise Qur’an? Such a woman is even more
unlikely, in sha Allah, to do such evil things, and is more deserving of
being well thought of; it is hoped that her prayer and the other good things
that she does will keep her away from such shameful deeds, and we hope that
by His grace, Allah will open the door of guidance to her, because of those
good deeds, and she will begin to wear hijab as prescribed in Islam. 

Moreover, the hurtful things that they say about her in her absence are
haraam; in fact this is a major sin, and if it is untrue, then it is a
grievous lie. What people must do is fear Allah with regard to the honour of
the Muslims, especially women and girls, because the impact of speaking ill
of these people is immense, and she may be mistreated all her life because
of something that was said against her out of animosity and enmity, and the
one who said that word, or transmitted it, or approved of it, will bear the
burden of that sin. 

Allah, may He be exalted, says, warning against such talk (interpretation of
the meaning):

“And those who annoy believing men and women undeservedly,
bear on themselves the crime of slander and plain sin”

[al-Ahzaab 33:58]. 

The Messenger of Allah (blessings and peace of Allah be upon him) said: “Do
not envy one another, do not artificially inflate prices, do not hate one
another, do not turn away from one another, do not undercut one another. Be,
O slaves of Allah, brothers. The Muslim is the brother of his fellow-Muslim.
He does not wrong him, let him down or despise him. Piety (taqwa) is here” –
and he pointed to his chest three times. “It is sufficient evil for a man to
despise his Muslim brother. The whole of a Muslim is sacred to another
Muslim, his blood, his wealth and his honour.”

Narrated by Muslim (2564) 

Please see also the answers to questions no. 66699
and 112134 

And Allah knows best.