Not quite sure what you mean by "how do I lose her..."
Any context? Are you being paid to take photos for someone? Objectively, is this person 'fat'? Or do they just 'look fat' in your photos? Did they tell you "Please make me look thin in the photos"?
At a certain point, you must figure out if you took an unflattering photo or you've already did your best and the person 'looks fat' in the photo. Some people may get offended/insulted if you photoshop them too far from reality... If I were in your shoes, I'd either show them what I have and have a frank discussion about what they would like to tweak (if any), or schedule a reshoot (but please make sure you figure out how to take more flattering shots first!).
You cannot make an orange look like a banana, lens won't lie, and cannot lie. Best you can do is capture her slightly better angle, but helps if she's dressed in her fav outfit and at least she'll be happy