I guess I am in the minority, but I LOVE any kind of horsey gifts. I still remember my cousin (very non-horsey) bought me some horse treats one year. That's been like 15 years ago and I still remember it! And when my foal was born, a neighbor gave me a foal halter for him (I didn't have one yet and she actually went to our local feed store, talked to the lady that owned the place, and the lady had told her I was looking at foal halters but didn't buy one). How thoughtful is it for someone non-horsey to buy me a horse gift? I think it is VERY thoughtful!
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.