Just going to set this up so you understand the situation a bit better as whole.
Also, this is going to be long. So get a cup of coffee or something and chill for this one :)
At the time, I was 23, my GF (Samantha) was 24 and her best friend (Lucy) was 23. I had been with my GF for around 3 or 4 years at this point. At the time, our relationship was rocky at best. I stuck around because I didn't know really what life would be like without her and I didn't want to find out. I had developed some major trust issues with her and really everything was a ticking timebomb. I knew that, which is why I had no qualms about what happened that night.
So yea, I met my GF pretty much the first night of University and she, along with the other girl struggled to meet new friends for whatever reason. Lucy lived in the University Halls the room opposite mine, so I was able to get her and my GF to start hanging out together, so they became best friends, at University.