Does she have a horse herself? Maybe get something useful like quality riding gloves + a bottle of wine?
My YO doesn't drink so I'm getting her a box of chocolates then (not sure what the right name for it is) one of those things that holds a haynet open to fill as she was complaining she needs an extra hand to fill haynets and it will be useful to everyone!
I usually get plenty of alcohol and chocolates they are very welcome last year one owner also gave me some really good hand cream which was very useful and much appreciated. A haynet holder would be very helpful, I might drop a few hints as its a job I hate but will probably get one for myself, brilliant idea