I would geuss she's saying what people have told her, that he's bored. Sometimes with a chronic rearer there really isn't always a "reason" for him to rear. I have a chronic rearer. He rears when he doesn't want to work, when he's done with his trail ride and wants to go home, when I've been standing too long and he wants to get put away, He'll even rear when I'm on the ground with him. For the most part it's disobedience as Kevin says. With mine there is no pain and definitely no fear, I've had the horse for almost 23 years. I have never been able to "cure" his rearing. The one thing I have never tried, and will never try, is to flip him over. I don't believe in that.
My suggestion is first to rule out any pain. Make sure your saddle fits correctly and that you don't have any teeth issues. Now, I am assuming you've ruled out fear because you are saying that the horse is "bored". Which means he's not afraid. That makes me think you are dealing with a disobedient horse. He can be a kind and loving horse and still be disobedient...
With rearing you first need to learn to "feel" the rear before it comes. You'll feel it in the shoulders, there will be a tensing, or a bunching of the shoulders just before he go's up. BEFORE he go's up you need to move him forward. If he's moving forward, he can't go up. So keeping him moving. If you miss the sign and he pops the rear then you can yank the rein quickly to the right or left to knock him off balance, he'll throw his feet down, the instant his feet hit the ground you've got to kick him forward. If you don't get him moving forward then you'll end up in another rear. You can pop him in the head between the ears but the thing with that is, you HAVE to do it on the UPWARD motion. If you do it on the way down it has no effect. The thought behind a pop between the ears in mid-rear is that the horse thinks it's hit it's head, if you don't time it right then it doesn't work.
you can use a tie down but that won't fix the problem. I've got many other things that I've done but to be honost, my horse is 25, almost 26 and will still rear.
"Be a best friend, tell the truth, and overuse I love you
Go to work, do your best, don't outsmart your common sense
Never let your prayin knees get lazy
And love like crazy"