I would tell him and be honest. What's the harm if it is just a friend and if your boyfriend loves you and would love to marry you, right?

A healthy relationship is built on a foundation of trust, so I wouldn't try to hide this - then it will seem more suspicious if he finds out somehow or you may act strange around him knowing you've kept a secret.

He'll understand since your friend invited himself and his wife didn't even ask you if it was OK.

(((Hugs))) Don't fret.