I m afraid this horse is not 'broke'. Your 'very close' fried only trained her to be a 'pack horse' or a 'packer' as we call them. This is a horse that only goes with another horse. They have been trained to stay with their 'herd' under saddle like they would in the pasture. This works great for pack horses and dude string horses. They can't be ridden away from the string which is what keeps dudes safe when they do not know how to ride. They faithfully carry a pack or dead elk and stay right behind the guide or lead horse. Most pack mules are only trained this way. A good rider could not ride them away from camp or the other horses if their life depended on it. [Think of the string of gentle saddle mules that go to the bottom of the Grand Canyon and back.]
If it were me, I would probably call this 'good friend' up and tell her exactly what kind of monster she had created and you would like her to take this horse and finish training her and you would like to have her back once she has trained her to ride as well by herself as she does with another horse.
In reality, I doubt she can get this done or she would have done it in the first place. Anyone that would 'think' they were training a horse and they left this big of a hole in their training, is not much of a trainer.
As for just riding her through it like any ordinary barn or buddy sour horse -- probably ain't going to happen. There is such a huge hole in her training, that this horse needs to be backed up to learn a lot of things from scratch that she should already know.
You are also seeing the extreme of what can happen when you do not ride horses out by themselves. This is what we completely avoid when we DO NOT use a gentle horse to help a green horse go out into new places. We just start them out in the beginning and they are perfectly happy with riding anywhere by themselves. When we do ride with another horse, we ALWAYS make sure we split up and ride each horse by itself for a good ways.
If you can't get your friend to finish training this horse that you got stuck with, I will tell you what you should do next. Regardless of what she does, I would take this mare a long ways away from any friends and tie her up until she got happy. You will have to tie her in a safe place and you will have to leave her there for hours, maybe all day for 2 or 3 days. That will give you a start toward making her function independently. She is going to have to do that before she will even be trainable. When a horse is 'reactive' frantic, on the fight, they are not going to learn anything and you or the horse could get hurt. A horse MUST be in a 'responsive' mode and be receptive to training for anything positive to be taught. Otherwise, they just learn how to fight you better -- like this mare is doing. I read your response in my trail horse training article. You will just get more spinning and more than likely rearing or falling over if you really push the whole thing before she is better trained.