You can see it in 2 ways.
Firstly, she's getting the opportunity to ride your horse like in a lease so she should pay you (of course you shouldn't make her, just speculating)
Secondly, she's offering you a service so you should pay her.
Or the middle would be... nada!
Since she's a college student, I would probably give her a little something... because ya know, students are poor...
I would know.
I don't know how much money you should pay, as prices differ from area to area, but maybe buy her some horsey gift? Like a gift card for the local tack shop? Or for the barn so she can pay a lesson?
... Or just plain cash I suppose. Hmm, let's see what other people think. Though you said she loves Izzy, so my guess would be she didn't take this on with any money ideas in head, and if you offered some she would probably be too polite to accept it... or at least not without a bit of protesting and many thanks.