Can a Man Lead a Woman in Prayer?
Question

I am French and I recently embraced Islam. My question is about the prayer. I would like to know if one man is entitled to direct a prayer meanwhile there is only one woman behind him. In this case, the man and the woman are not married. So, I am wondering if this kind of prayer can be considered like a prayer in group (with more reward). Or, on the contrary is it misadvised or absolutely forbidden?
Praise be to Allah.If this woman is one of his Mahrams, there is nothing wrong with him leading her in prayer and it will be counted as prayer in congregation. But if she is a non-Mahram, and if his leading her in prayer means that he will be alone with her , then in this case his leading her in prayer is prohibited. 
An-Nawawi (may Allah have mercy on him) said: 
“It says in Al-Muhadhdhab: It is disliked for a man to lead a non-Mahram woman in prayer, because it was narrated that the Prophet (peace and blessings of Allah be upon him) said: “No man is alone with a non-Mahram woman but the devil is the third one present.” 
An-Nawawi (may Allah have mercy on him) said in his commentary: 
“In this context, “disliked” means that it is prohibited. This applies if he is alone with her. 
Our companions said: If a man leads his wife or a woman who is his Mahram in prayer, and is alone with her, that is permissible and is not disliked, because it is permissible for him to be alone with her in situations other than prayer. If he leads a non-Mahram woman in prayer and is alone with her, that is prohibited for him and for her, because of the authentic Hadiths that I will quote below, in sha Allah. 
If he leads (a number of) non-Mahram women in prayer and is alone with them, the majority of scholars said that this is permissible, and this was narrated by Ar-Rafi`i (may Allah have mercy on him) in Kitab Al-Adad from our companions. 
His evidence is the Hadith which we will quote below, in sha Allah, and because when there are a number of women together , a man cannot usually behave inappropriately one of them in the presence of the others.  
With regard to the Hadiths mentioned above, one of them is the report narrated from `Uqbah ibn `Amir (may Allah be pleased with him), that the Messenger of Allah (peace and blessings of Allah be upon him) said: “Beware of entering upon women.” A man from among the Ansar said: “O Messenger of Allah, what do you think about the in-law?” He said: “The in-law is death.” (Narrated by Al-Bukhari and Muslim). The “in-law” refers to the husband’s relatives; what is meant here is a relative who is not her Mahram, such as the husband’s brother, paternal uncle, cousin, maternal uncle, etc. The husband’s father, son and grandfather are Mahrams and it is permissible for them to be alone with his wife, even though they are also in-laws. 
It was narrated from Ibn `Abbas (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said: “No one of you should be alone with a woman unless a Mahram is present.” (Narrated by Al-Bukhari and Muslim). 
It was narrated from Ibn `Amr ibn Al-`As (may Allah be pleased with him) that the Messenger of Allah (peace and blessings of Allah be upon him) said from the pulpit: “After this day, no man should enter in secret upon a woman whose husband is absent unless there is another man or two with him.” (Narrated by Muslim)  
What is meant by a woman whose husband is absent means one whose husband is not in the house, even if he is in the same town.” (Al-Majmu`, 4/173, 174) 
Shaykh Ibn `Uthaymin (may Allah have mercy on him) said: 
“The phrase “leading a non-Mahram woman or women in prayer who have no man with them” means: it is disliked to lead a non-Mahram woman or women in prayer.  
What the author says requires further explanation: 
If there is one non-Mahram woman on her own, then to say that it is only disliked requires further explanation, if that means being alone with her. Hence it was interpreted in Ar-Rawd as meaning that the Prophet (peace and blessings of Allah be upon him) forbade a man to be alone with a non-Mahram woman. But we say: If he is alone with her then it is prohibited for him to lead her in prayer, because that which may lead to something prohibited is also prohibited.  
If there is more than one woman, i.e., if he leads two women in prayer, this is also subject to further discussion as to whether it is disliked, because if there is a second woman along with the first, this means that Khulwah (being alone with a non-Mahram member of the opposite sex) is no longer an issue, and if the man is trustworthy there is nothing wrong with him leading the two women in prayer. This happens sometimes in some mosques where the congregation is small, especially with regard to Qiyam Al-layl in Ramadan, when a man comes to the mosque and finds no men there, but he does find two or three or four women in the back of the mosque. According to what this author is saying, it is disliked for him to lead these two or three or four women in prayer. 
But the correct view is that this is not disliked, and that if he leads two or more women in prayer, there is no Khulwah and it is not disliked, unless he is afraid of temptation. If he is afraid of temptation, then it is prohibited, because whatever is a means that may lead to something prohibited is also prohibited. 
From the phrase “if there is no man with them” we may understand that if there is a man with them, then it is not disliked, this is clear.” (Ash-Sharh Al-Mumti`, 4/250-252) 
And Allah knows best.