He never dump Nishino, it was actually the other way around. He kind of regretted not speaking up and fixing up the problem with her that night. If you recall after the break-up, they met again and became friends instead. In Junpei mind, he thought she only wanted to be friends, even though they have a lot of development together.
Nishino is pursuing her dream to become a pastry chef and pointing out to Junpei that she will leave to French to complete this. You could say it was bit of her fault for not making it clear to Junpei, when he is very dense (and love vs. like).