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The real question is whether she is looking for help. What started the misbehavior, and what kind of behavior did it start as? You said that he's nipping and biting now, but has that always been the problem?

My main advice would be to cross tie him if available while grooming and tacking for the time being, or straight tie in a safe place if cross ties aren't available. If you think the issue is that he's getting more and more scared, then making it impossible for him to bite during grooming by cross tying him will allow him to build back that trust that he lost, since she won't be hitting him for nipping.

If the behavior is escalating, you should also encourage her to seek help from a reputable professional in your area. I can't really give too much advice not knowing anything more about the situation, but someone close to you should be able to evaluate everything going on.