Well, one idea I've had is a pretty risky one to try, but if it plays out well, it'd be awesome... the next time she comes and asks if she's getting fat, grab a little piece of their side and jiggle it rapidly, while saying, "Nope, you feel just right to me!" as they're giggling helplessly. The tickler gets to tickle her, and she knows that because they still want to tickle her, they're still attracted to her.
Yeah, it's a dangerous set up, but I think it'd be cool if it played out like that.