Break it down.. if lessons average $30. And trail rides (pay to ride) are average $20.
So between $25 average to physically ride a horse.
25* (3 days) = $75 per week = $225 per month
If she's going to go cheap, might as well charge something logical. I divided my horse's board by 30 days (1 month) and got the price per day plus a small vet savings fee.
That's before I put training on Sky.. now the price increased since he's better than a tie and ride since he knows his stuff.
All of it went towards Sky's monthly maintenance costs.
But that's just my opinion.
What do you think?
Now if this is just for a "I'm being nice and want to let you exercise" then she still needs a contract and your mom is very nice.
Actually, folks, exercise riders in my area charge the OWNER of the horse $25 - 30 / ride. So if this person is very experienced I have to say it's a good deal for the mom, if not, oh well other way around.
I'm unfamiliar with a part-board situation, however I part leased my boy earlier this year for a month and we split his board evenly, so the leasee's paid $125 and were allowed to ride 3 days a week. Therefore, whoever the girl is who is going to be riding him is getting an excellent deal and your mom is getting the horse ridden for very cheap.
My advice would be to be sure your friend is experienced enough to exercise a senior horse without supervision, does she have experience with horses or is she just someone who is willing to jump on him and go? In the best interest of the horse, you should determine that first.