I would not let this person near your horse at all, even if the instructor sends her over. I would talk to the instructor and tell her what happened so she is aware and ask that this girl is not sent to you again. Let her know you appreciate help while you are learning, but it upset you the violent reaction from this girl.
The biggest mistake a newbie can make is to baby the horse, Sure it can be a pet but you do have to be firm and make the horse do as you ask. A pampered unruly horse can be dangerous. I see no problem with treats, just remember treats by their nature should not be regular or expected and never use food to catch your horse.
If the horse was actually biting a bit of a slap on nose might be justified and you can't help but give bit of a yell (I got bitten on the upper arm a few months ago, it is still vivid in my mind :) - but screaming in the horses face as you described is over the top. To my mind, this sort of behaviour from the girl is born of fear, but I am speculating here.
I don't think your horse did anything wrong and if the girl didn't like it a flick of the hand would have discouraged the horse. She has caused more harm than she prevented as now you have to regain trust for bridling. Get someone there that you trust to guide you in putting on the bridle a couple of times and make sure it is adjusted properly. Videos will help also. You will be fine, just try it. If the bit bumps the teeth it wonít hurt unduly, any more than yourself if you catch your teeth with a spoon.
Just because you are lacking experience does not mean you are stupid. You do not have to feel inferior to these more experienced people. Itís a bit like when youíve had a baby, the whole world has advice and a lot of it conflicts Ė you take what suits you and ignore the rest, if it doesnít work try something else.
When you are not working with your horse watch what others do, you can learn a lot that way.
You donít have to allow any tom, dick or harry to bully in and push you or the horse around. If they are any good they will tell what to do/or what they are going to do and why. It is still your horse.
So I guess it worked? Is it all fine then? But it was kind of traumatic, if I'm honest. Watching him flinch, watching his eyes roll around. He has never acted like that, or been treated like that while I'm around. I felt like I couldn't say anything because A. I'm new and don't know anything and B. Was in shock.
So during my lesson, I was riding him and he was super happy as usual. The head teacher came over to tighten a stirrup and I asked him about slapping. He said sometimes if a horse is doing something really bad, a strike can teach them not to do it.
But I've seen him do it while training a new horse he's working with. He hits it, yes, but not crazy hard and not on the face. And not with crazy anger.
No it is not fine. You were made uncomfortable and intimidated. You felt defenceless as to what to do when you felt protective of your horse and powerless when she came back with her crazy anger again.
YOU DO NOT HAVE TO ACCEPT THIS . . . donít accept this. No-one should be doing anything to your horse without your permission, and they should be explaining anything they do.