I agree with this. I rode a big Percheron/TB cross for a while, and he only started kicking/bucking at the canter when arthritis came into the picture for him The solution for him was a cushy retirement, NOT having multiple riders come out to ride him am have him put his leg through walls.
Of course you can ask or even tell but the descriptions given of the owner don’t make it seem like they are the most responsible of folks

Or, if you don’t want to bother, insist that the horse does not have arena access and say that you understand if they want to move the horse.
I think this is a good idea. She didn’t use the arena the way you asked her to (repeatedly) so she and her horse lose the opportunity to use it. She will probably either leave or stop using her horse as a riding horse either of which will solve the problem for the future. I think it is also reasonable to ask her to pay the cost of materials and a handyman to do the repair. This is not normal wear and tear if it is frequent behavior on the part of this horse and she has ignored requests to ride away from the wall.
I myself had a horse who went through a phase of being a wall kicker. My horse turned out to have ulcers, and treatment solved the urge to kick but for a while there was also a habit to deal with. Riding away from the wall for a while (the rest of that winter) was fairly simple to do.

She is just good at acting nice.
This is truly a brilliant observation.

Maybe in the future create a contract that all part-boarders need to be approved and they need to sign a contract. Not just a liability waver but including the rules of the barn and penalties for damage.
Yep. No one should be riding at your farm without at least a waiver. When they sign the waiver, you can talk to them individually and let them know that the horse CANNOT be ridden on the rail, or whatever you decide. Without a waiver on a unpredictable horse, you may have a bigger issue on your hands if the rider falls off and gets hurt.
Honestly, I’d let this boarder go. It’s too much risk to your property. Something else likely WILL happen.
The chances are that your other boarders aren’t having a great time riding around this poor horse and it’s many and varied riders, either. I suspect there would be a collective sigh of relief if you asked her to move on.
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Everyone must have a signed contract, and lessors must sign a waiver that includes they will abide by barn rules.
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You probably won’t get a dime from her if you ask her to pay for the arena
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If you allow this horse to stay (and I don’t think you should), it is banned from the indoor. If the weather is too bad to ride outside and the horse becomes fractious without work, she can come out herself or pay you/your staff to hand walk/longe the horse in the indoor. This is non negotiable. “If turnout or outdoor arena are unavailable, Owner agrees to exercise the horse personally X number of days per week. If Owner is unable to personally exercise horse X number of days per week, Owner authorizes Stable to exercise horse X number of days per week at a cost of $X to be invoiced to Owner.” (I’m not an attorney but when you hire one for your contract, this would be something to ask them to add).
4 If it were me (and it has been), the conversation would be “Either the vet comes out to do a full lameness evaluation and treatment for any diagnosis, or you need to leave.” I refuse (as should you) to watch painful horses be ridden on my property.
I trust your assessment that he hurts. I’d want him off my property. What happens when he’s had enough and bolts or bucks someone off? He’s being ignored.
I like Demerara’s suggestions in general here, but worry she’ll skimp on doing what the vet would prescribe. I just feel for the horse, but his owner isn’t picking up what he’s telling her nor what you’ve told her.