Polypenguin, I don't see at all how you could interpret that nycindie suggested you were a serial killer. It seemed very obvious she wasn't implying your friends were either, just that you might think these friends have good qualities but it doesn't mean they don't have qualities bad enough that it would tip most people towards staying away from them regardless of the fact they are enjoyable company.
Anyway, I don't think it was your girlfriends job at all to go talk to them about what she heard. I don't have to confront people that I get bad vibes from or hear say rude things. If you wanted to talk to them about it that is your business. I'm not quite clear if you've dropped the friendship but I know I'd be uncomfortable if I was in this situation and my husband was still wanting to be involved with people who had acted like that.
Happiness will never come to those who fail to appreciate what they already have.