As someone that’s had two young horses die suddenly (albeit 17 years apart), I’d say you’re spot on. A hand written note, some wine (or something stronger if that’s what she’d like), and flowers were always appreciated. I hate to ask, but if the horse didn’t make it, maybe also a lovely picture frame or gift card to a company that makes horse hair jewelry (if you know for sure she has hair). If it’s appropriate, maybe wait a few days and ask if you can come by to be with her. Some people I didn’t want to see, but two (horsey) people flew across the country to be with me and we just sat on the couch, drank, and watched crappy tv together. Having someone around that really gets it and was happy to just be there and do whatever was helpful - having someone around that wants to talk about the incident or the horse, not so much.