[What should a person do when someone unacceptable steps forward to lead?]

Answer: The solution is that he intends to pray individually (furādā), recites for himself, and stands and sits with them; his outward following of the imām and waiting for him does not harm him. When the imām gives salām, he gives salām with him, and his prayer as an individual is valid.
This is mentioned in the marginal notes on Sharḥ al-Azhar according to the school, except that they stipulated that his praying along with them must not amount to suggesting and misleading others into thinking that the imām is someone whose leadership is acceptable. If such deception occurs, then the prayer is not valid—unless he is able to remove the deception before the time (for that prayer) ends. This is the meaning found in the marginalia.
I say: Dispelling the deception ought to be with respect to those who prayed behind that imām following this worshiper’s example—such that were he not to pray with them, they would not have prayed behind him.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1