To be fair, she may just be toying with him because she knows he's a pubescent boy who's starting to have these fantasies. Plus, the older him isn't looking back on it negatively, so I don't think it's fair to condemn them outright when the situation may be a little bit more complicated...
Though, I hope they actually didn't do any sexual stuff and this is a bildungsroman that explores their relationship. I don't think she's much older than her early twenties, and I'd even say she could be 18-19 at a stretch and having a romance story about a kid who falls in love with an older woman who's a young adult and is trying to come to term with adult responsibilities that the kid is unfamiliar with could be a very compelling narrative as they both seek to find their way in the world. I also hope he's flying back to find her and get married, but I don't know for certain if that's true.