I'm pretty much with the consensus on this one. I'd have to draw a line with her on this one and let her know that one woman and one man at a movie is a date - even if they are "friends" and she needs to choose if she wants me in her life or not. If she insists on going, she's made her choice.
I'll add in that I'd probably have a talk with the dude too. Sometimes you just have to draw a line and let the guy know where things stand.
Dr where my wife worked quite a while back was a bit of a player. He never made a direct hit on the wife, but there were times when she said he would invade her personal space, put a hand on her shoulder (not around her, but pat the shoulder closest to him) or try to give her a back rub and such. She had told me this would make her really uncomfortable. We'd interacted with he and his wife as couples and he and I had hunted some, been to his house, seen his karate pics and such. One day we're out hunting. While out, I take the opportunity to point out to him that if I ever had reason to believe that a guy was making a move on my wife, preacher or not, it wouldn't matter if the guy had a gun, knife, or black belt in karate I'd beat the crap out of him. He kinda turned white, gulped, and changed the subject.
He kept a professional distance from that point forward though, and we were still able to remain friends. The wife didn't know until just a year or two ago when I finally told her about our talk.