Sounds like the horse may have a dominant streak and almost certainly respect issues. I have a gelding that was guilty of some pretty similar unwanted behavior. For him, head tossing was an attempt to get the annoying human off his head before he escalated to rearing. For me, the day he reared was the last straw. He was with a professional trainer within a week.
For safety's sake, suggest she get her horse working with a trainer, and respectfully decline to handle her horse until she has done so. A good trainer will be able to determine if the rearing is motivated by respect or fear or if it's a conditioned response and address it accordingly.
If you're ever in the same situation again, do not back up. Get out of the striking zone by moving sideways. In the horse world, backing up is a sign of submission, so if you back up the horse thinks he can push you around. I have heard of it suggested that a rearing horse can be corrected by pulling down and sideways, then making it move its feet once it has all four back on the ground. Getting the horse to move its feet can also be a matter of respect - the one who moves his feet the most is more submissive. The more you can make the horse move his feet without moving yours, the more respect you'll have.
I am happy to report we had a major breakthrough with my gelding in the respect department just yesterday because I finally learned to hold my ground and demand respect from the most challenging horse I've ever worked with. I owe it all to my trainer, and it was worth the budget adjustment. No more worrying about him trying to run me over! Let her know a good trainer can make a world of difference for her horse and everyone who interacts with him.