I'm in a really awkward position. My best friend is cheating on his girlfriend, and my own girlfriend is telling me I need to speak up and reveal his secret. No idea what to do, and I'm disappointed in my friend, too.
That's horrible. I'd talk to your best friend and tell him how you feel firstly.
You're right. I'm worried about having to possibly lose my best friend. I'm worried about having to stick my nose in other people's personal affairs. I'm worried about my girlfriend being mad about me being involved in it. It's all just a big hassle.
If your best friend is truly a best friend of yours, they won't leave you over this. It isn't right to tell his girlfriend in my opinion, but it is good if you speak to him about the situation.