Cowboy, I'm not talking about rough or strict, I'm talking about trying to take away someone's self respect and mental abuse. Being called a bitch is just that, abusive.
The minute I read the op's statement that he has no desire to learn because 'her way of doing things is boring', my reaction was, "Your ignorant behaviour is boring, why don't you leave now?". Then when she said he escalated and called her a bitch, that's the final straw, no man who respects a woman, let alone claims to love her enough to marry her and spend his life with her, would call her a bitch. That is a serious red flag and would get him tossed in this house.
I just as firmly believe that a woman who loves her husband and respects him would never call him names either, and if either one of them does that, it's unacceptable.
He's obviously already had at least one failed relationship (9 year old son, where's mom?), and I bet he talks about that woman like she is dirt beneath his feet.
I would be very rough and strict with that horse, and I'd be very happily doing it alone without that fiance. No one has the right to take someone's self respect, but I'm also very cognizant of the fact that if you don't give it to them, they can't take it from you.
So my basic premise was, take back your self respect and I'll bet a whole bunch of other things improve in your life, starting with the handling of that horse which ought to become a whole lot easier when she doesn't have someone undermining her training and handling.
My prediction would be, if she doesn't tell him to hit the road, the fiance will continue to tease the horse until she bites him or kicks him severely and then she'll come home to either find him beating the crap out of the horse for what he caused or he'll be screaming that the horse is crazy and needs to be gotten rid of, or the horse will hurt the 9 year old because of what the father is teaching him to do. None of those scenarios bodes well for the future of the horse, and NONE of it is the horse's fault.