Don't let her be alone with him. I'm afraid he'll do something more than just throw her on the bed. I would be with her when she breaks up with him as well. It would probably help to talk to him, and maybe there's a school therapist/regular therapist so he can talk to them.
Phones are a wonderful thing. It sounds like moth needs time to work up the courage to break up with Shorty. She can record a couple of the tantrums without him knowing -even voice recording while the phone is in her pocket would work. Then, when she's ready to break up with him, she needs to get all of her stuff away from him and go to everyone one-on-one and explain things, with proof that she's telling the truth. The important thing here is that she does NOT offer ultimatums, because then she will lose everyone as friends.
If she explains something along the lines of "Me and Shorty are breaking up, and I'm not going to make you choose who to be friends with. I just want you to know my side, because I'm scared that I'll loose you as a friend, and I don't want that to happen." Then explain the story with the evidence.
Also: She needs to make sure she has copies of the evidence. Preferably with a copy on your stuff that Shorty doesn't know about. That way if he tries to throw a tantrum and damage the phone, it won't destroy the evidence. This might sound like paranoia, but you never know with guys like that.
If I believed that therapists can actually fix this guy, I'd say he needs therapy. I quite literally mean FIX HIM because there's something wrong with his venting. My partner tends to vent in a similar way sometimes, to the extend of say, throwing his cellphone to the floor. It's not healthy, but it's not punching walls and doors for the drop of a hat either. That's bloody teenage behavior.
It's been said here before: she must break up with him. Some men just tend to vent that way, but if my partner would ever throw ME instead of whatever of his own things he has near him, it'd be over in a heartbeat. I love him very much but that's just how it is. Even if it's happened only once, it's only likely to happen again later. It might be painful for her to do so, even though he's obviously an ass, she might still love him. But it is as it is: this is for the sake of her own life. And later she will thank herself for ending this.
If she's worried about his reaction - which we all are at this point, I think - she could start by very slowly taking her belongings away from his place. But chances are that he'll notice this. It's better that she gets some help from you and other friends to get it all out there in one trip, then break the news, possibly with you and other friends by her side. (Assuming he's only such a violent ass when they're alone together. If he loses it then, at least she won't be unprotected and others can see what kind of a guy he truly is.)
And honestly? Losing some friends over this isn't a big deal. In fact, if they can't support her for leaving an abusive partner, they're hardly friends at all.
She said she's already slowly removing her belongings from his house and returning them to hers, bit by bit, and has been doing this since (I think) before the most recent "big" incident. At least it's a start.
I'm definitely going to suggest having SOMEone around when it happens -- at least somewhere else in the house so that if shit starts to fly, they can hear it and help her remove herself from the situation. Hopefully it'll all go well, or as well as it possibly can.
Lesson learned- when you are not over or through with a previous relationship, don't rebound into another. Things do not usually end up well and your choices are usually clouded by wanting to move on in your life too quickly. Sometimes you just need time to heal before you can move on.
She did wait quite a while (I suppose, I guess I really wouldn't know what a "good" amount of time between relationships is) before jumping over to Shorty -- about a year, to be fair. Perhaps it still wasn't enough time.
First thing's first. She needs to cut contact and she needs to cut it now. There is absolutely no question at all as to whether or not she should break up with him. He has shown himself to be violent, manipulative, destructive, and possessive. It will only get worse.
When she breaks up with him, she must NOT be alone. He will react poorly. Make sure you are with her or nearby. She must not break up with him in private, it must be in a place where there are MANY people. If it is in private, he may react violently and two or three people might not be able to control the situation. Furthermore, in public, he cannot prevent her from leaving and if he chooses to follow or harass her, there will be help nearby.
Contact must be completely broken. If he is as persistent as you say, he will cause problems afterwards. If necessary, she can file a restraining order against him. This may mean giving up some friends, but if he is as batshit insane as you make him out to be, they'd have to be crazy not to see through him eventually, particularly after the breakup.
I know Shorty is a friend of yours and you feel like you should defend him a bit, but that behaviour is completely out of line and there is no need to protect him. He's abusive, plain and simple, and people defending him only enable that behaviour. Please support your friend and let him be. He deserves no sympathy and had it coming.
I'm glad to get opinions on whether or not other people should be around for the break-up, at least in the vicinity -- I was worried that if she did it privately shit might go down, so I'm happy that other people are agreeing that it should be done with some back-up around to help cool things off or get her out of there.
I do admit it feels a bit... not mean, but weird I guess, to be vilifying Shorty, but believe me when I say I do know that his behavior is completely wrong and that I need to stand by Moth during this.