Definitely a bra would be an "iffy" gift, unless she expressed interest in one. Like other folks, I have to try on a bunch before I find one I like. I finally got some that fit me better last year. I tended toward the droopy ones, too, but before I had a breast biopsy, I was told I needed certain kinds. As JimRat said, maybe a gift certificate to a store that sells bras and other things. Or go the pendant route, if you actually want to give something tangible.

My mother had a friend who gave her underwear for Christmas, and the card was signed by the friend and her husband. My mother was really surprised. She said she wasn't comfortable with the husband being involved, plus evidently her friend had been sizing her posterior up and decided what size she needed! They actually fit! We still laugh about it!