FMF?

I had an interesting talk with a married girlfriend of mine. She was saying that she was looking into "spicing up" her and her husband's sex life so was looking into a bringing someone else into the bed, so to speak. I told her it would be a great way to spice things up but it seems that a FMF is the order of the day, and she was wondering if I'd be the other person. I was surprised to say the least as I know the larger lady isn't everyone's cup of tea and they're both quite slim. However she said she knows I get tested and whatnot. Apparently he doesn't know either, it's going to be a surprise. I told her if she wanted to go ahead with it I'd be interested.

Thing is, I guess I should have asked more questions. Does she want us two to do anything for example?