That does seem like an overboard reaction from your friend. When I read your comment, I thought of the phrase "it's not over until the fat lady sings", which isn't any kind of slur, but a tongue-in-cheek reference to opera.

I think some of the "reaction" to people calling someone fat (or skinny) is positive. It would be good if we all learned not to shame people for their size. (I'm not saying that you were doing anything of the sort - you obviously weren't.)

It sounds like your friend isn't very discerning and has decided that any comment about anyone's weight, EVER, is bad. Sometimes you just have to let things slide off your back, especially if you try to explain and it's not accepted.