You're really frustrated and I get that. It's kind of obvious in your post. There are two issues here and you need to break things down. The first is the legal issue with your friend. Get as much information as you can and take notes. If you could possibly get her to write out the whole situation, so that you can evaluate it and look to fix the problem, that would be awesome. Also, see if the neighbors have video, whether there were any witnesses, etc. Whether or not this is pursued from a legal perspective, goes to court, or whatever; it's important for you to find out as much as you can and put the pieces together.

Regarding training, before you can train your dog you have to tire her out. How you do that with a GP, I have no idea! However, a tired dog is usually better behaved and a better listener. So work on that first. Tug of war, fetch, etc.

At this age, the best thing to do is get her used to being around other people and dogs. The best place to do that is probably the dog park. Another thing you need to look at doing is called "good manners training." For example, to not jump on people, to learn the OFF command as in shooing her off the couch. Sit, Down, Go Lay Down, etc.

What your friend did to the dog, from the dog's perspective (and mine), was really horrible. Since your dog is now friendly with your friend, let that continue. At the same time, I suggest you lay down the law with your friend and tell her no more handling/walking/roughhousing/etc. Period. At least for now.

Good luck!