I'd be totally mean and and just keep her going "Yeah...still looking, can't find anything" and if she got REALLY annoying I might be even meaner...though I'm sure you don't really want to know just how mean I can get -chuckles-
I have a friend (only one really, and she's off to korea for a year on monday x.x) who basically 'used' me to give her FREE lessons...so I can really feel the whole 'using' you for your horse thing. I wouldn't have minded except even after 2 months of one lesson a week I still had to tell her how to mount, not to let the horse eat while working (in halter, in bridal, under saddle) and honestly won't ever do that again. If a friend wants to ride my horse, then they better know how to ride, or pay me to take a lesson since while I'm on the ground, I'm clearly not on my horse, and my time is precious lol....
You can also 'milk' her a bit and offer her ONE day around the horse after a week or so of chores.
Again....I don't 'always' take the high road -shrugs-