Most Helpful Guy
You're a naughty little girl aren't ya? Naughty, naughty!
Most Helpful Girl
If you don't tell them now you will probably just keep feling guilty about it and end up telling them in the end.
It sounds to me as if you like your friends husband better than your flirt so you might want to consider ending it with your flirt. If you do that, then there is no need to tell him, since it will only hurt him unnecesarily.
As for your friend, she deserves to know. Even though it will hurt her a lot.