What Nan and Plank said. If you don't know anything about horses, for god's sake do not buy one for her! It's not a breathing car. It's like a 1,500 lb golden retriever that's stupid.
And realize that owning a horse is roughly like paying for a child living in it's own apartment. A child that takes 5+ pound shits and 2+ gallon pisses on the floor. A child that needs custom made shoes every three months. A child that can't go to the doctor, that you have to have a specialist come to see.
Oh, and if you think taking a dog for a walk twice a day is a drag, imagine needing to go attend to the needs of an animal that lives off site, EVERY DAY, no matter what the weather or how you feel.
You really like her, be smart, buy GF a nice horse sculpture or painting, seriously.
Oh, and y'all know how I got a seriously messed up back? At least a part of the reason is because of horses, i.e., a couple of bad falls while riding as a child (and, just like a motorcycle or bicycle, EVERYBODY falls, sooner or later).