Is music and singing forbidden (haram)? What is the ruling on accepting the prayer led by someone who works as a music teacher and a singer, and is it permissible for him to lead the prayer?

The scholars have unanimously agreed on the prohibition of musical instruments (ma'azif) and entertainment tools, and the opinion that they are permissible is weak. Your friend's argument that Allah would not have given him the talent for musical instruments if they were forbidden is invalid, for Allah tests His servants, and he must repent to his Lord. As for his prayer, it is valid if it fulfills the conditions and pillars, and praying behind him is valid according to the preponderant opinion.