OP, is there a specific reason why the owner is leasing the horse out? Can she no longer afford to care for the animal, or is it simply for convenience? That can make a big difference.
On a slight agreement with SpeedRacer, not that I thought you sounded arrogant at all, but if you ARE paying all the expenses of horse ownership, why not purchase an equine of your own? Can save some drama.
However, if it's the initial purchase price that is the issue, then perhaps just suggest to keep the back feet bare for now, and then slowly bring the idea of completely barefoot back up when the owner can see that the horse can handle life without shoes, and that the difference in expense is well worth it.
I'm actuall surprised that the horse is allowed in pasture with back shoes on. I know a lot of barns that restrict shoes to the fronts only for animals that will be sharing pasture space with other horse. Or is he in a private paddock?
If the horse owner is relying on your lease for the well-being of the horse, then I would think it not out of reach to suggest the barefoot method. If the owner is simply being courteous by not charging a lease fee, and you just like riding that specific horse, then I would probably just leave things as is.
Would be awful, though, if you ended up having to pay the vet bill for an injury caused by shoes. :/
The horse was moved to the barn of my choice when the lease began, and i am paying all associated costs as far as board, feed, vet. all of my own tack and equipment is being used. the owner of the horse is being allowed to keep his trailer on the property for no charge, and i also paid the key deposit for the owner for his key to the tack building.
the only cost that is still on the owner is half of the cost of farrier. that works out to about $35 a month, assuming an 8 week schedule (what the horse has been on) for farrier.
i have brought the owner's horse expenses down from the $300 range per month to $35 per month.
the owner cannot ride due to injury, has had some 'extra work' dry up lately and has reduced income, and does not have time for the horse either. horse has not been ridden in 'a few months'. he is trained for reining, but i will not be pursuing that. pleasure hacking, light to medium arena work, and training him to ground drive this winter (yes, i have the owner's approval for this). not hard work at all.
the horse is out in pasture with a number of other geldings. i too worry about him being shod in back with others around, even with his disposition being fairly quiet.
i am a single parent with one job, one income. i cannot afford the initial cost of buying a horse in the near future, as much as i would LOVE to! i did own for 10 years, so i know how much nicer it is to have a horse you can 100% call your own. i am getting back in to riding after a 12 year break. not sure if i am quite ready for the commitment of owning yet, so leasing is my option. the month to month expenses for my lease situation are no issue for me, neither are any unexpected vet bills.