"hey he said this and i know he's a kid but that was inappropriate" or something so that it's not making her seem like a bad mom or the kid seem like a bad ass child. kids just say wild things at times to get a reaction out of ppl.

at the same time, it could be something he saw or heard or experienced and you don't want to write that off in case it's a sign (example the girl across the street from me always liked playing house and being the dad and she'd talk about sex all the time, come to find out, she had been touched by some male family members and the way she talked to us was the way she was talked to. basically it was some "if you're my friend you'd do this" type mess).