An anonymous note just gives him the chance to deny and be more careful about who he shares info with.
If the info is specific enough he'd have a hard time denying it.
Although I dont think the OP knows enough information in this case to be specific.
I find this so often - women in denial even when you are told your DH is cheating. If someone sent me a note and a name of a woman my DH was cheating with ... their motives are not the issue. The truth is what I want.
You rather blame the messenger and bury your head in the sand.
The content of the letter is what I need to focus on. I'd either do my own covert investigation or confront.
Many people don't want to get involved apart from to let you know your spouse is playing away. That's not cowardly at all. Why get embroiled when you can simply pass on the info with verifiable evidence.
I'd be terribly uncomfortable with my DH going away with this guy. How do you know his friend's GF isn't going as well? And bringing her single friends along? You don't.
It's a great cover story though. The poor wife will think he's off on a lads trip with Tom, Paul and Nick. Not knowing any the wiser.
So often people talk about a mates affair and they are up to no good themselves. Affairs can seem like so much fun and when all the friends see him glowing after great sex, they could decide to give it a go. They will alibi each other and cover no matter what.
Every one of us have our own mind, but I've known affairs to start after a friend sees another living it up and having a whale of a time getting away with it so easily.
It's great that you have no secrets and he would tell you this, but I'd keep wondering why - especially as you know her.