Like i said, you should confront him. Tell him how his actions made you feel. Now, be prepared for lots of verbal judo (examples of that in my previous post). He will accuse you of not trusting him, if you tell him you found evidence on his Facebook of "liking" all of her posts and the event, and how you put 2+2 together and then his actions at the music venue of leaving you to go introduce himself (he should have brought you with him and introduced you as his girlfriend).

He will do his best to keep the focus on your snooping, and off of what he did, so he doesn't have to answer to you for trying to meet this woman in a way that isn't suspicious. Don't let him take the focus off of himself. Don't let him deflect.

It's clear as day that his plan now was to meetup with this woman in person, and use the music venue and event as a convenient way to do it covertly.

It's not as though he was going to admit "hey, honey, I have this hot female friend on FB I want to meet in person finally, and I see this upcoming event as the perfect way to do that, you won't mind would you?" because of course you'd mind. You're his girlfriend. So, he went about it in a rather sneaky way, but you're smart and you found out.

So, yes you definitely need to address this with him. If, after a year, he pulls the verbal judo on you that I mentioned previously, and shuts down and gets defensive, be a good Bridget Jones and walk away from that Daniel Cleaver and go find yourself a nice Mark Darcy instead.