What should I answer the girl's family if they ask me about a young man whom I know does not pray, commits indecencies, and falsely claims repentance?

The person consulted regarding a suitor must mention his flaws, because that is a matter of trustworthiness and obligatory advice, and it is not among the forbidden backbiting. Rather, it is among the permissible backbiting for the sake of benefit and warning. The فقهاء (jurists) have stipulated the obligation of this, even if one is not consulted.