As expected, happiness can't last forever. There's some other stuff in-between, like them getting into a fight over him treating her as a replacement for his deceased daughter, and her finding a place in school in the art club, but I'm skimming to the end here. They get found out, guy goes to jail for a couple of years, demonised by the media as having taken advantage of a young girl. But before that happens, they go on a road trip together which is poignant in its own way, probably because of the guy's dead daughter. Guy tries to commit suicide at the very end, but she saves him.
In the final chapter, a reporter is doing a piece on the case, talks to a bunch of different characters who were involved in the story, and meets with the guy towards the end (he was released after about 2 years). He's going on a road trip, can't remember why, but probably to do some soul-searching. He's basically lost all his ties to friends and family because of the incident, and he also still wonders if he did the right thing with the girl.
It's hinted at that the girl is also doing well, although I'm not sure exactly what happens at the end, if they really do meet again at the beach or if that's just an ending scene the author came up with. (In the final few pages, she might be headed to the same place as him, just via train, not sure.) The reporter's piece concludes that well, legally there's no question about it being right or wrong, but beyond that it's more complicated of course. And also that it is because we have injuries that we are able to heal, and she hopes that is the eventual end point for these two scarred souls (more or less).