I'm with the boyfriend on this. If she doesn't want to do anything in the winter and he wants to do everything in the winter then it will cause problems. If he goes out and takes part in everything winter has to offer she'll probably get mad because she feels like he's ignoring her. He'll probably get annoyed because all she wants to do is lay around the house in the winter. She probably told him about her winter hibernation routine when they started dating. 

People seem to think he's trying to change her when all he did was talk about how she wants to be a homebody during the winter. What if she is trying to change him if she starts wanting him to stay home with her all the time? That stuff goes both ways. They should probably both move on.