An adult messing around with a 15 year old is gross and awful and shouldn't be taken lightly. I'm not sure why you're convincing people to be sympathetic because the adult isn't an old man but it's no less dangerous or concerning or illegal.
1. I called the guy scum. I’ve repeatedly referred to him as trash. I’m hardly trying to convince people to be sympathetic towards him.
2. 18 years old is a legal adult in some countries, but not in all countries. Age of majority varies. Whether or not he is a legal adult (and I legit don’t know if he is - I’m not sure what country they live in, only that it’s not the USA), 18 is also still considered physical and mental adolescence.
3. It’s not about him not being “an old man”. It’s about the fact that she is a 15 year old and not a toddler or elementary school child. Pedophilia is, by definition, the sexual attraction to prepubescent children. Ephebophelia is the term used for adults who are exclusively attracted to early-to-late adolescent. But we have no evidence that he is either one of these things. The only evidence we have indicates that he is a sleeze ball, bad boyfriend, and total trash. There is a difference.
4. It might be gross and awful -- but it is less dangerous and concerning. It also might not be illegal. I don’t know where these people live and I’m hardly a lawyer. Even in the USA, there are variances in state laws on how to treat statutory rape when both parties are teenagers. And even if it is illegal, enforcement can vary pretty significantly.
Facts matter. Context matters. This guy is still a creep.