We know Paul was unfaithful, and Beatle or not kind of makes him a jerk (as most men are), and we have no evidence that Jane was.
If you have another boyfriend, then you are being unfaithful, she'd had another boyfriend, so she was unfaithful too. Again, obviously he was MUCH more unfaithful but I think she was too and I think it's actually quite likely it was just being confronted face to face with Paul's infidelity(apparently), as opposed to just hearing about it and seeing a picture, that caused the final break up. I mean she obviously knew well before they broke up - there are actually pictures of him with other girls on vacations and things like that, so unless she was living 24 hours a day with her head in the sand she knew and she chose not to break up with him over that for quite a while.
It could not even be that, she may have just decided she wanted to break up and the infidelity really had little to do with it, it was a just convenient reason. Like he said sometimes you really click and sometimes you don't. Sometimes it's just convenient or interesting and so you stay together and go through the motions. She never let going out with him crimp her style, she worked when and where she wanted, did what she wanted, so it's not like she was sacrificing much by being his girlfriend during those years.