Well, taking sex and relationships out of it, if Nicole feels that the MC isn't going to be goal oriented and driven like her she could break up with him for not being the man she thought he was. There doesn't have to be cheating on anyone's part, simply growing apart and going their separate ways. She is one of those people with a two year plan, a 5 year plan, and a 10 year plan. MC seems like someone who only has those plans because she drafted them for him.
If Nicole even kisses another man there will be loud cries of "NTR!" and accusations of how they were betrayed by the development team
As I've said before, using cheating as a plot devise that causes characters to break up and, perhaps, for Nicole to exit the narrative completely would be interesting. But I don't think some members of this community would tolerate that. You could also just have her randomly die in a car accident or something
Good luck!!