You're both focusing on a superficial issue - the matter of surface appearances. You're being dragged into these dynamics no matter how much you downplay the matter.
What she has is a poor self image and serious issues about her body. She really
needs counseling. Your support and dialogue with her while commendable, won't overcome this as it's not just a minor problem. It's good that you want to help and you're trying. This is deep-seated distorted view. I'd stop short of calling it an obsession.
Worse still is that she thinks that your reaction to certain physical appearances of women means something to you more than what it is. She needs to learn to trust you..and others. But that will be pretty hard because she has a distorted view of herself and the reality at hand.
Secondly, this sentence made me drop my jaw:
Seems like such a childish issue to be having considering we are both middle aged now
What makes you think that late-'20s to 30 yrs is middle-aged? Depending on who/whom you talk with, 40's to mid-40s is generally considered middle-aged, give or take a few years.