So what to say? (also is it *really* that bad to have fast food once a month?)
I work with a registered dietician who completely backs you up, if that helps (and I didn't even have to ask--she has said so many times on her own). If the rest of your diet is healthy, then occasional fast food (especially if you order wisely and in moderation) will not make you unhealthy, nor will it make you gain weight.
Your friend, on the other hand, does
sound kind of unhealthy--maybe this is some kind of competition thing?
"I like having lunch with you, but when you say those things, it really
bothers me. I've already explained my reasoning to you, and so I feel that you don't respect my ability to think my choices through. Can we agree to disagree, or should I stop seeing you at mealtimes?"
"You seem preoccupied with food and fat. It must be scary to think that certain types of food might make you hugely fat all at once. Have you considered talking to a professional about this?" (This may sound snarky but it's not. Her comments send up legitimate red flags.)
"Want some?" then chase her around with it until you've burned the calories she thinks you've ingested.
Okay, maybe just try the first two.