Is the mini kept in dangerous conditions? I hate to suggest it, but with a broken fence and she can't even walk out there to care for the mini, it could very easily lead to an animal abuse case. It's heart breaking, but if she loves this animal at all, y'all just need to have a very serious conversation about what's best for the horse, and consequences of not achieving that. If she's not mentally capable of making the best decision, then never mind physical cabality, you need to act on it. It happens to everyone, eventually they just can't take care of themselves, let alone amyone else, and they need to accept it. I know I'm being harsh but reality is harsh, don't drag an innocent animal through that.

If you're able to arrange for keeping the horse, there are alternatives to companion animals, I've read a goat or chickens are good for keeping a horse company/amused.