Is congregational prayer valid and permissible, and does it have multiplied reward, if a man leads a single woman who is not his wife in prayer?

If the woman is a mahram to the man, there is no objection to his praying with her, and it is considered a congregational prayer. However, if she is a stranger (ajnabiyyah), and his praying with her necessitates khalwah (private seclusion) with her, then it is haram, as Al-Nawawi said: "What is meant by dislike (karahah) is the dislike of prohibition (i.e., tahrim), and this is if he secludes himself with her," citing the Prophet's (peace and blessings be upon him) hadith: "No man is ever alone with a woman but the third of them is Satan," and his (peace and blessings be upon him) saying: "No one among you should be alone with a woman except in the presence of a mahram."

If a man leads strange women (ajnabiyyat) in prayer and is in khalwah with them, the majority of scholars have affirmed its permissibility due to the absence of khalwah when there is more than one woman present, provided there is no fear of fitnah (temptation). Sheikh Ibn 'Uthaymeen clarified that the dislike (karahah) disappears if a man leads two or more women in prayer, unless he fears fitnah, in which case it becomes haram.