What is the ruling on the prayer of one who led a person who cursed the religion, and then repeated his prayer individually, and is he thereby considered to have doubted the disbelief of the disbeliever?

The questioner is not considered to be doubting the disbelief of the disbeliever. However, he sinned by failing to forbid the person from evil. His duty was to explain the ugliness of his action to him and invite him to repentance. He does not need to repeat the prayers he performed with him.