The guy was either looking for:
1. an underage girl, or;
2. the youngest legal woman he could get his hands on.
In any case, he played with fire.
Perhaps, she said she was 18 and maybe he didn't know that sex work is off limits till 20.
Hell, maybe she even said she was 20, and she may have looked it.
Who knows?
Nonetheless, this is not a game you should play and he has to blame himself, mostly.