You've known your girlfriend for 7 years. So she knows you as well as anyone, apart from your mother.
You tell her something she really needs to know. She goes into a catatonic state, weeping and wailing and gnashing of teeth. Then goes to sleep with a friend.
She knew already. (Face it, she did).
This performance is over the top. For anyone.
I suggest the best thing for you to do is carry on as if nothing has happened. You've told her, she had to be told. The rest was just a silly tantrum.
The only question is this friend. Is this friend the sort who will either let her talk it all out and help her get her thoughts in order? The sort to support 'you guys'? But ultimately, will support her, whatever she decides? Or the sort to use any opportunity to attack you and suggest she leaves you? (In other words, a looser who doesn't want her friends to do well until she does, which will be never?).
I ask this point because your girlfriend will know what sort of person she is.
I suggest a number of points.
1. You had to tell her. She had to know.
2. Her behaviour is silly, childish, contrived and immature.
3. If she says she wants to end your relationship, then this is just an excuse. She intended to end it anyway, but was waiting for a point of maximum impact.
I may be completely wrong. This is the impression I get from what you have said.