I have also had a friend buy me a running martingale one time (which I don't use) but she included the receipt and said I could exchange it if I wanted. I exchanged it for a very nice bit which I still own. So yeah, I can pretty much name all the horse related gifts I've gotten and to me they are the BEST kind of gifts.
You almost can't go wrong with a nice halter and lead rope. What you can also do, if you don't mind doing that sort of thing, is include the receipt and say that if it isn't something she can use she can exchange it for something else. Or better yet, maybe a gift certificate to a favorite feed or tack store? I ADORE feed/tack store gift certificates. But maybe I am weird that way.