Well if Amane cheated I wouldn't say that but, I'm guilty, I know I will still hate Nene even after it is explained why she is a bitch to Amane.
Although we have seen the way Amane thinks about her and I wouldn't say that's how a person who cheated would think about their ex, but this is fiction and real life so anything could happen
I will not say Amane is the bad one here because obviously not but her attitude of not letting Nene go is still somewhat chilldish because if you consider that Nene is really over Amane (which she obviously not) then having an ex you will still consider as a friend is still trying to make a move on you is somewhat obnoxious.
But here it's somewhat ok because Nene does that for whatever reasons unknow just to try to cut the ties with Amane. For me it's pretty obvious that Kou will try to put them together and will say no in fall.