You need to gently remind her that MFP is not for fat bashing (of any sort) and if that kind of talk continues you may have to remove her. You need positive reinforcement as opposed to negative. Is it so hard for her to say "Good Job!" and leave it at that *despite* what you may have actually eaten?
I would go private and when she asks why, you can have the conversation with her. You don't need that kind of thing on MFP. (I'm Leighish on MFP if you want a cheerleader
Oh wait! I think you ARE my friend on MFP!