Women often gain weight in a relationship. I think its because they feel secure and loved unconditionally. Which says a lot about you by the way. Thumbs up.
Its a difficult decision to make though. You will be judged by her friends and family if you break it off with her now.
And he will have to admit to himself that he is maybe shallower than he thought he was. All of this is OK. There is no shame in liking what you like. If you no longer find her physically attractive, then it would be better for everyone involved to break it off earlier rather than later. She will then be free to find someone who is OK or even happy with the fact that she is now a (probably) 100kg woman.
There needs to be a discussion though. Sometimes people gain weight due to life's circumstances (40kg though...), and maybe she will learn to love an active, healthy lifestyle.
I have seen women gain weight early in their relationships, then as normality sets in they manage to get their routines in order and lose weight again.