Is it permissible to pray behind an imam who performs ruqyah on women individually in his closed office, and there have been many rumors about him assaulting them?

It is not permissible for a doctor or a practitioner of legitimate ruqyah (exorcism/healing through Quranic recitation) to be alone with a foreign woman (non-mahram). What this person is doing is forbidden (haram), and the people of the town should not remain silent about it. As for the assault on women, it requires verification, and he should be advised to abandon being alone with women. Prayer behind a persistent sinner or an innovator (Mubtadi') is valid, though disliked (makruh), unless his sin is an act of disbelief (kufr). If people abandon the mosque because of him, then his matter must be brought to the attention of the mosque's supervisor to arrange for an imam whom the worshippers are pleased with.