One thing to add to this is that it's possible the horse may have some pain issues, too. Don't get me wrong, I am in total agreement that this is very dangerous behavior and needs correcting immediately. Should never have gotten this far before being taken seriously.
But, it's possible that there is some kind of lowgrade and constant pain that is making the hrose have a very sour and defensive attitude to begin with. An ulcer or a very bad misalingmnent of the poll or pelvis, a sore back due to an illfitting saddle, or kissing spines, can make a hrose very crabby. The way the OP described it, his demeanor changed from sweet to angry. So, this is something to think about. Horses can't tell us they are in pain except by acting out.
But, I do agree that the behavior must not be tolerated.
The OP mentioned that she corrects his behavior in the crossties. But, if she is actually correcting it, then it would not recurr. What might actually be happening, and it might be happening in other intereactions, is that she nags on him. She snaps the lead and maybe he stops for a sec. Or, she verbally upbraids him, and he stops for a sec. But, he 's back to doing the No-no thing in just a bit, and NOT A THING has changed. Except that the horse sees such corrections as no more than irritants. A fly is an irritant, and a horse will swish it away, or bite it away. And that may be how the horse is viewing this owner; and irritant.
It may be that she has never really seen how a good trainer will address some bad behavior, and with what force and clarity, persistance and patience . . Until a real change takes place. So many folks, and I can include myself in this, just kind of go through the motions of disciplining our horses, but we aren't really doing what it takes to make a change. That way we wouldn't always have to kee revisiting the same darn issues , over and over again. For some hroses, revisiting these things in the half hearted manner I just described drives them out of their gourds, and they react with complete disdain for the human.