What if this happens to you?

2 and then 1. Like no s***, imma confront! I hate betrayal and my friend deserves to know the truth no matter how painful it is

depends on how close i am to the friend, if not getting involved could be tricky

If they're not a close friend, I'd ignore.

Hmmm this is tough. I would tell my friend

i'll tell my friend, with backups to show

2 then 1...see his/her reaction first...

Investigate first... Don't be stupid

Gather evidence, then tell friend.

I will hint if is close friend.

depends. either 1 or 2