I would just go up to him and ask him where his girlfriend is. That way the girl he's with would definitely know he was taken.

I wouldnt get involved. SOMEHOW, you'll end up the villian. They will direct their anger at you. I know ya wanna help, but stay outta it.

I'd probably go up and ask him "So, where's *insert gf's name here* tonight? I was looking forward to talking with her." and see what he does and how he plays it off.

As for that really dumb girl I never have to worry about my hunny's brothers covering for him because he and my blood brother are in the same fraternity and my brother would *kill* him if he ever pulled a stunt like that (or anything close for that matter).