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

An uncovered woman asked a young man to take a picture of her with a camera, and she was not Muslim, and he agreed to do that. Is what he did permissible or did he do something that was very wrong, and what advice do you give to people like him?.
Praise be to Allah.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.