I can't say what I would do, I can say however what I would like to do. I'd act like I'm okay with it. I would then test the waters to see if I could do the same with her best girlfriend. If I could I'd make sure she catches me as well. If not I'd make sure she catches me having a threesome with 2 girls! Then we could both go on with our relationship knowing we're even, hopefully. Coz if we break up we'll still be thinking of each other, missing each other and moping over each other. So what's the point of breaking up?
Murder my best friend, then get answers out of my girlfriend, and based off how she responded and felt would determine if I'd stay. I care about my girlfriend a lot and love her to death so if she did that to me, I may give her a second chance.. not the wisest decision, but every now and then people may make stupid mistakes..
At the very least I would leave. Once a cheater, always a cheater. Never forgive someone who is disloyal and sneaky. Open relationships are different, because they are being straight forward and honest. They are simply experimenting or enjoying consenting sex that both partners are aware of. I just hate sneaky ass people I guess, it's not even a matter of jealousy.
I would be so pissed and we'd be through, both relationship with friend and girlfriend. They are both obviously bad for my life and I won't have it. I don't need to throw any fists , maybe some harsh words, but that's it