I prefer the term "fat". Like your friend, I'm good with that.

But I also like the term "zaftig", just because I like that word and no one has ever called me that!

There are some words that are neutral, but have been so heavily abused by the people that invented them, that using the word is like walking into an emotional minefield. Like most things, the best in the world you can do (which you just did here - thank you!) is simply ask the group of people you're talking about, what they would like to be called.

And then oblige them.