As far as I know, a 14 year old with a twenty something isn't even that unusual in Thailand (which doesn't mean that I think it is OK). I doubt the police would get involved unless the mother makes a complaint, which is also very unlikely.
It's just another thing that we foreigners, while bothered by, can't do a thing about, even if it's "against the law" in Thailand. Certainly preaching to parents or trying to influence the child is pointless.