Four year relationship in jeopardy, I really need some advice!?

This is a terribly long story but I'll try to sum it up as best I can. My boyfriend & I have been together four years, we're in our early 20s now & the relationship has been pretty stable. We've also had a very close friendship with a girl friend of ours whom he'd known for a few years before I met him. At the end of March this year we all attended a party a friend of my bfs was having. My bf had to leave early so my girlfriend & I stayed. I was driving & had a couple drinks early in the night so I'd be ok to drive home hours later. 4am rolls around & I'm ready to go but she wants to stay. A guy who lives there is a total creep and has been making pes at her for a long time so I was super apprehensive about leaving her there. I don't know why I didn't drag her out of the house but in the end I slept there with her. I was up all night panicking because I knew my bf would be mad, which he was. We had a huge fight over the issue. There's more to this, I'll add....