I like the framed photo of her horses idea. If there's so much land around you, maybe you could sneak out to where her horses are & photograph them in their element? Early morning & about 1 hour before sundown are the best times, light-wise for photography. Zoom in or get in close. Click, trill, squeal softly or talk low to get their attention on you if you want, (most animals will perk their ears up & turn in the direction of a higher pitched noise) or just have them grazing, mutual grooming, running, whatever. Take looooots more pics than you think you'll want. Then have 1 or 2 printed, and frame them. I love it when someone has done that for me.
What about a gift basket of things you can never have too many of? Feed buckets, halters, hoof picks, hay nets, brushes... Or vet-type things that are always nice to have on hand? Vet wrap, ointments, scissors, syringes, ripped & rolled sheets for wrapping, etc.
Or something personalized for her horses? A stall nameplate or halter nameplate.
Something impractical & extravagant she might not indulge in for herself? Really nice shampoo & conditioner, something beautiful, I don't know! Head down to the tack store & see what you'd love if you had horses but didn't really ride, and would never actually spend the money on. Kinda hard, I know.
A book about horses. Something maybe like "Chicken Soup for the Horse Lover's Soul" or something light. Stories more than "how to".
A tee shirt that has some kind of horsey thing on it from the tack store (or ebay--probably cheaper!)
Homemade treats for her horses. Just dump horsey foods together (I use oatmeal, flax seeds, molasses, chopped apples & carrots, some oil & an egg or two til it's the right consistency--depends on if you want cookies, muffins or "cake"--, put into muffin tins or drop like cookies on a cookie sheet or spread in baking pan, bake til set). You could serve them in a feed bucket tied with a ribbon.