I agree with Rugbug, but depending on how the schedule is, I think the WS may be there a good deal more than 3 hours. Unless she’s throwing lunch hay instead of dinner hay.
I think there has to be a balance between setting a fair rate for the tasks that are being accomplished, and being realistic about how long the WS is at the barn.
If she’s there most of the day, $35 probably feels like SHE’s the one being taken advantage of. If she’s really only working 3 hours, then it’s pretty fair.
Either way, I don’t see this situation as unfair to the owner. Doing stalls twice, feeding/sweeping twice, helping with turnouts, is not a tiny amount of work.
Also agree with Rugbug - it’s your responsibility to teach her/expect the stalls to be cleaned how you want them to be. Don’t put that on her. Unless, of course, you’ve tried and she still isn’t doing them up to par. In that case, don’t complain about it, find someone new. Very simple.