Do You Tell a Friend You Think His Woman Is Cheating?

So this happened:

So far, I've got about 80 responses.
Here's what some said:

• confront the girl and guy
• you tell him right away
• let his girl see you and she will panic on her own
• take a photo
• take a photo then send it to friend right in front of them
• take a photo and then group text it to both of you
• confront her and tell her to call him before you do
• act like you didn't see anything, then next time the three of you are together bring up seeing her and let it play out
• blackmail her for money, then when you get it, tell him anyway!
• tell me who it is and I'll tell him in front of her
• Call her up -- obviously she's easy LOL!
• Ask him if he'd like to know
• I would not get involved... not my circus, not my monkeys
• damned if you do damned if you don't

So what did I REALLY do? I texted her about it... and she hasn't responded yet.