my gf wants her friend to watch...

I have never masturbated in front of a GF's friend. But I've masturbated in front of more friends and roommates than I can count and it never affected the friendship or living situation. However, that's because I always made sure the other person was OK with it before I let them see me or did it in front of them. I've told roommates they didn't even have to ask before coming into my room while I was masturbating if they had a question or needed something.

The closest thing to the actual topic at hand that i've been party to was the couple times a friend and I rubbed one out to nude photos of his girlfriend (also my friend.) I would like to be clear, the girlfriend had posted the photos online to a message board kind of like this one where there were often threads calling on members to post their nude selfies. So, I don't feel like he was betraying her trust or anything. For all I know, he told her we did it immediately after that first time.

She was very sexually open. She often masturbated with the boyfriend or one of our other female friends when people were over at the boyfriend's house partying. To be brutally honest, I've tried to find those photos a few times over the past 20 years and really regret not saving them to my hard drive when she first posted them.