An hourly rate isn’t wrong and if you figure you’d be there all day, $10/hr x 10 hours comes out to $100 a day. You could easily be worth more than that or the day might be longer or shorter. Then there’s whatever expenses you might have to get there (mileage and/or meals and/or hotel).
If there are a lot of horses and you are skilled, very fair to ask for double that. Either way your pay is on top of your expenses. IMHO.
Part of the question too is are you doing the trainer a favor mostly for her benefit, or are you getting a day of learning and exposure that you will personally value from doing this? Are you thinking, Yes! This is a fun way to spend my day or are you taking time away from other things you need or want to do, like riding your own horse or your regular job or the like.
It might change how you feel if they are her horses than if they are horses that will have their expenses billed to clients.
Horsemen tend to underpay for these kinds of services. Up to you if you are good with that or not.
I would probably ask her what she was thinking of paying and then negotiate from there, but also have in my mind what I needed from the day. It doesn’t have to be cash, if cash is a problem; you might be able to swap for lessons for example.