J.B.
If it doesn't bother him, ignore it and know that she's just a small, petty and insecure person. At 14, he's old enough to "get it" and understand that she's petty and to be ignored.
If he is telling you that it bothers him, and he's not comfortable talking to his dad about it directly, then it's OK for you to help facilitate that conversation. My younger sons are 11 and 13 and when my ex does something that bugs them, I tell them to talk to him about it. If they're really upset though, and want my help, they'll tell me that and I'll broker a conversation between them and their dad (who can be childish and petulant). If my ex had a significant other who also said or did things that bugged them and they weren't comfortable talking to him about it, I'd help them with that conversation as well.
But if he can let it roll off his back and doesn't care, then let it go - it seems that he's probably figured out that he's the bigger person (compared to your ex's girlfriend) already.