The horse being a stallion and normally a gentleman doesn’t necessarily mean you need new barn help- any horse can get a wild hair.
If you know the animal rears, stud chain over the nose (I prefer a rope lead- I find them easier to grip than flat leather, and certainly not nylon) and wear gloves. Keep the horse on a short lead, just long enough that you’re not applying pressure on the chain, and long enough for you to get out of the way. Walk with purpose. Cheekiness- jigging, throwing the head- gets a growl, crowding gets a pop on the shoulder with the free end of the lead. If the horse goes up, get out of the way of the front end (back towards the shoulder) and use the lead to bring the horse around towards you. He must immediately walk on. Trot forward if you have to if the animal sulks at the forward aid, which in this case is going to be a cluck, a growl, or a pop of the free end of the rope towards the hocks, but the second those front feet hit the ground that sucker needs to be moved forward. Rearing is an evasion of “forward” and they can’t go up if they’re moving on.
If you have the horse in the paddock and are taking off the halter or shank and that’s when the animal goes up, get out of the way of the front end (towards the shoulder if you can to hold on, which is easier if you know the horse is going to do it) and, if you have to to preserve your skull, drop the lead and get out of the way. You can catch the horse later. Look, if the horse habitually rears when you’re taking its halter off, that’s a serious problem that should be corrected by a professional, but we’re talking about standard barn help here- who hasn’t had a frisky horse get a little carried away at morning turnout and start trying to play before his halter is off? Or have a fieldmate provoke him? The best-mannered horse on the ground will occasionally pull a wingding out of sheer joy at being alive, and right now I’m talking about that horse, not the confirmed bad actor. It doesn’t sound from the OP like this horse is a habitual bad actor. I don’t care if it is a stallion, being intact doesn’t exempt a horse from having manners and it sounds like this one normally does (“perfect gentleman”) so maybe he was just having one of those mornings.
So the next time this girl leads this horse, she puts a stud chain on, wears gloves, wears her helmet, marches the animal towards its destination, and probably this “perfect gentleman” won’t do anything and she’ll have been prepared anyway.