The best way is to be open and to listen to her (if someone has a strong belief about their body image especially, it is very difficult to persuade them otherwise) It is possible that she may be bulimic (but no info here lists those symptoms)
You may gently point out to her that the fact remains that he had left, that you do not see what he had seen (a problem with her weight) and ask her if she thinks that perhaps since her ex was not able to be honest with himself and focus on his own reason he had used her as an escape goat and dumped the responsibility onto her.
In reality, no other person can make someone do something and on some level she ought to know that he had done what he had done due to his own choice.
What I worry about relates to the experience I had with my ex, where she did gain so much weight. I never complained and in fact didn't see her as fat. ( my ex wife was a very beautifull woman and did in fact gain a tremendous amount of weight ) My ex, later blamed me for "letting her get fat." Now while I realize that a woman's weight gain is not my resposibilty, I worry about, being so supportive to my girlfriend, that she won't have the desire to lose the weight that she wants. And then later, be unhappy because of that. But then I worry about encouraging her to do her diet, thinking that that will support her fears that she needs to lose weight to maintain my love.
Everything thus far about our relationship is healthy, but this is something I worry about simply because of the experience with my ex.
In either situation you can not be fully responsible for what each woman choses to do or not do. You may just have to let things go and see what comes out of it and try not to worry much because worry won't change anything except make you stressed out and she can detect that (and mistakenly may assume that it is because of her)
Just try to accept that each person will do what they believe will make them happy and satisfied and it is ultimately their decision. It would be helpful to focus on something else instead and just let her deal with the diet at this time.