If you are already involved with girl 1, then you would be cheating on her if you start seeing girl 2 at the same time...you can't have your cake and eat it too without reprocussions. Girl 1 sounds VERY insecure but girl 2 sounds like she is a loser as a friend(sorry, but if she was my friend, hitting on my boyfriend, I think I'd knock her out then I'd never speak to her again..she is NOT a friend and she has no loyalty) so, as far as their friendship, it must not be strong AT ALL or girl 2 would not betray her friend.
Anyway, I think if you want to pursue a relationship with girl 2, you HAVE to break-up with girl1. Would she be more upset if you broke up or cheated on her? (BTW...that threat of killing herself is just a manipulation tactic that so many people use..especially men, but they never really mean it) I think she'd lose her mind if she found out you cheated on her with her so called "best friend"..trader..betrayer. You also have to remeber, what goes around comes around, the wheel turns and if you choose to cheat on your girlfriend instead of being honest and breaking up with her, someday, you will be completely in love, only to find out you've been cheated on....it never fails Kharma is a "you know what"...there's a reason someone made up that saying.
