Okay, so a situation went down not too long ago that involved me, my girlfriend, and her friend. Her friend was the kind of person that wouldn't let her leave her (the friend's) house until a few minutes before her curfew. Well about two weeks ago I was at my girlfriend's house and her friend came over about 20 minutes after I had gotten there (we all live in the same neighborhood) and wanted to play just dance with her. They went up to the guest bedroom and her friend locked the door so i couldn't come in and when my gf unlocked the door to let me in her friend didn't want to dance because "they didn't have the fan in the room and she would overheat" (I kinda call bs because I can wear two jackets in 90 degree weather and break a light sweat). So I sat in the room and my gf went to go and get the fan. While she was getting the fan her friend sat in the corner of the room and was on her phone until after she got back. After my gf got back with the fan the friend said that she didn't want to dance anymore, soon after she went home. Not even ten minutes later she blows up my gf's snap saying that she doesn't like me or being around me and even tried to tell my gf that she thought she (my gf) didn't really like me either and if that was the case she should break up with me because "that's not okay". She continued on about how I'm creepy and i make her uncomfortable and tried to say that she thought i made her (my gf) uncomfortable too. She then went on to say that i hit hear really hard, acting like i bitch slapped her, then stated it was on the palm of her hand, which did not happen as I am not a violent person. After all of that she pulls the "I've been your friend since you moved into the neighborhood" card and tried to make here choose between me and her (the friend). She ended up choosing me, her friend hasn't messaged her on snap or anywhere else since she blew up on her, and my gf said she likely won't message her because of the way she acted. I feel bad because she was the only friend in the neighborhood she had left other than me. At this point I'm just writing this to get it out to someone because I need to tell someone but the problem is no one is up to talk about it. If you have any ideas as to what i should do please reply.

Hey.

I think that you should not feel bad about it. Obviously, I can't read the mind of your girl's friend, but maybe.. just maybe.. she was jealous, because she was afraid that you'd end up replacing her (like you ended up doing). Your girlfriend shouldn't have to pick between her and you. Do you think that it is possible to talk to your girlfriend's friend in any way? I think an honest conversation could be a solution (not a harsh one.. be gentle). Try to understand her. She is just another person, just like you and me. If things stay as they are, nobody is happy. You should ask her why she would make your girlfriend make such a tough decision and if she thinks that it would be okay to try make a friend leave someone they love.
Tell your girlfriend's friend that you don't want them to be separated and that it doesn't have to be this way. Maybe she ends up hating you a little bit less and you are able to get along. And maybe nothing changes and your girlfriend has lost a friend.

In the end.. it's up to you if you try and change the situation.
You can do this.

You don't have to feel bad. Friends shouldn't force friends to make decisions.