A woman asked him to take a picture of her and he did that

The Muslim has to respect the rulings of Islam and be proud
of its teachings, and he should be a caller to the guidance of Islam by his
attitude more than his words, especially when he is among non-Muslim
people. 

Righteousness is a honour by means of which the Muslim rises
in status, a light that shines among mankind. It is not a burden or
something shameful that he should be embarrassed to show openly, let alone
go against openly. 

It is well known in Islam that it is forbidden to look at the
faces of non-mahram women unnecessarily, and it is even more haraam to look
at their hair and whatever of their adornments are visible. 

Allaah says (interpretation of the meaning): 

“Tell the believing men to lower their gaze (from looking
at forbidden things), and protect their private parts (from illegal sexual
acts). That is purer for them. Verily, Allaah is All‑Aware of what they do”

[al-Noor 24:30] 

It was narrated from Ibn Buraydah from his father that the
Messenger of Allaah (peace and blessings of Allaah be upon him) said to
‘Ali: “O ‘Ali, do not follow one glance with another; the first is allowed
but the second is not.” 

Narrated by Abu Dawood (2149) and classed as hasan by
al-Albaani in Saheeh Abi Dawood. 

It is obvious that taking a woman’s picture with a camera
involves looking closely at her face and form, along with the talking,
chatting and haraam smiles that are also involved. So how about if the woman
is uncovered, wearing makeup and showing her adornments? 

The basic principle is that the Muslim should denounce evil
and forbid everything that goes against the commands of Allaah, and he
should not keep quiet about it – let alone help with it and take the matter
lightly.  

The Prophet (peace and blessings of Allaah be upon him)
said:  “Whoever among you sees an evil action, then let him change it with
his hand [by taking action]; if he cannot, then with his tongue [by speaking
out]; and if he cannot, then with his heart – and that is the weakest of
faith.” Narrated by Muslim (49). 

It would have been better for this young man to take the
opportunity to call this woman to Islam and tell her of the virtues and
purity to be found in hijab, and tell her that righteous believing women
from Maryam (peace be upon her) to the Mothers of the Believers and the
believing women were women of virtue who observed hijab, and that Islam
protects women and regards them as precious jewels which are not permissible
for everyone to see. Perhaps Allaah might have guided this woman at his
hands and her becoming Muslim would have weighed in the balance of his good
deeds on the Day of Resurrection. 

This young man has to repent from what he has done, and not
do such a thing again. You have to advise him and remind him that he should
keep away from such actions, and that it is better to openly follow the
rulings of Islam in front of non-Muslims who he hopes will be impressed when
they see the greatness of this religion and its teachings. 

I ask Allaah to guide us and you to that which is good, and
to teach us and you guidance, truth and faith. 

See also question no. 1774. 

And Allaah knows best.