Okay, I'm going to gear this towards Dressage -
These are the some of the big online equestrian shopping sites:
Bit of Britian
Dover Saddlery
SmartPak
Back in the Saddle
Dressage Extensions
Dressage riders are all about bling - rhinestone/crystal/beaded/metal browbands are huge as are embellished spurs and spur straps such as these:
https://amateurequestrian.com/wp-con...02518176_o.jpg
You can Google any of those items I mentioned and probably find them on Etsy too.
Saddle pads are always nice - I have a YUGE collection. My favorite are sheepskin like Mattes pads but they can get pricey. That said there is nothing wrong with a nice quilted pad which are easier to maintain than sheepskin plus you can get them monogrammed on most of those sites above.
Tack aside most horse people like equestrian decor. Again, most of those sites have everything from paintings/prints to jewelry and statues. Another idea might be equestrian wool hook throw pillows. Wayfair has a ton like this:
https://secure.img1-ag.wfcdn.com/im/...row+Pillow.jpg
Next up, if she lives somewhere with cold winters and she still rides you might consider cold weather clothing such as insulated riding gloves and equestrian themed fleece/hats (no scarves! Big no-no around horses. Dover sells them but again they shouldn't be worn around horses). If she lives somewhere warm they also make cooling shirts and the like that are quite popular.
Last but not least if they keep their own horses (have pastures) you could get the horse a giant ball.4
https://youtu.be/PSww-wPqtgw