Can he be alone with a woman to lead her in prayer?
Question

                Is it permissible to be alone with a woman in order to lead her in prayer?.
Praise be to Allah.It is not permissible for a 
    man to be alone with a woman who is not his mahram. 
The Prophet  (peace and 
    blessings of Allaah be upon him) said: “No man should be alone with a woman 
    unless there is a mahram with her.” Narrated by al-Bukhaari (5233) and 
    Muslim (1341). 
And he  (peace and 
    blessings of Allaah be upon him) said: “No man is alone with a (non-mahram) 
    woman but the shaytaan is the third one present.” Narrated by Ahmad (178) 
    and classed as saheeh by al-Albaani in Irwa’ al-Ghaleel (1813). 
These ahaadeeth are general 
    in meaning and indicate that it is haraam for a man to be alone with a 
    non-mahram woman under any circumstances, even if that is for the purpose of 
    prayer. The scholars have stated that this is forbidden. 
Al-Nawawi said in 
    al-Majmoo’ (4/174): 
Our companions said: If a 
    man leads his wife or a mahram of his in prayer, and he is alone with her, 
    that is permissible and is not makrooh, because it is permissible for him to 
    be alone with her when not praying. But if he leads a non-mahram woman in 
    prayer and is alone with her, that is haraam for him and for her, because of 
    the saheeh ahaadeeth which I shall quote below insha Allaah … then he quoted 
    the ahaadeeth that we have mentioned above. End quote. 
It says in al-Mawsoo’ah 
    al-Fiqhiyyah (19/267):  
The fuqaha’ are unanimously 
    agreed that being alone with a non-mahram woman is haraam. They said: No man 
    should be alone with a woman who is not his mahram or his wife, rather she 
    is a non-mahram, because the shaytaan may whisper to them when they are 
    alone, inciting them to do that which is not permitted. The Prophet  
    (peace and blessings of Allaah be upon him) said:  “No man is alone with a 
    (non-mahram) woman but the shaytaan is the third one present.”  And they 
    said: If he leads the non-mahram woman in prayer and is alone with her, that 
    is haraam for him and for her. End quote.