Is it permissible to pray behind someone who exaggerates in their love for the Prophet's family and insults the Companions, and is prayer behind them valid?

Slandering the Companions (may Allah be pleased with them) can either lead to disbelief (kufr) or to grave sin (fisq), depending on the nature of the slander. Praying behind someone who commits the type of slander that does not lead to disbelief is permissible, though disliked. It is preferable not to pray behind such a person if another imam is available. As for the type of slander that leads to disbelief, it is not permissible to pray behind the one who commits it, such as someone who slanders the honor of the Mother of the Believers, Aisha (may Allah be pleased with her). Indeed, praying behind an innovator whose innovation amounts to disbelief is not valid.