Answered by Ustadh Salman Younas
Question:Can a man lead a non-mahram woman in prayer, in public or private?
Answer: assalamu `alaykum
It would not be permissible for a man to lead a non-mahram woman in prayer in a secluded place where both would be alone. This is isolation (khalwa) with the opposite gender, which is strictly prohibited.
As for leading her in a public place, this would be permissible. However, proper decorum cannot be emphasized enough in such a scenario and if there is any fear of wrongdoing it must be avoided.
[Ibn `Abidin, Radd al-Muhtar]
Checked & Approved by Faraz Rabbani