My .02 cents...
We always operate on the "cold light of day" philosophy, meaning any concerns we had we would address upfront with both of them there...the 4 of you. We spend some time before, just the two of us, discussing exactly how we feel about the situation and then exactly how to best voice our concerns and be as low key as possible so as to try not to upset anybody.
That always helps us to clear out any preconcieved notions on our part or theirs.
No doubt there isn't any way to be certain someone is as ready to swing as they may or may not think they are, but it will help open the door to some honest communication.
When M and I break new ground so to speak in our own swinging experience, we discuss it in the cold light of day and if we are uncomfortable talking about all the details of the experience we figure we'll have some regrets afterwards. Based on that we decide about going forward.
So in this case, if things went right and everybody seemed OK, I'd point blank ask something about how she would feel seeing her boyfriend with you or whatever. Talking pretty explicit about sex with someone's partner (in a respectful way of course) can conjure up emotions and give the person a reality check so to speak. Just my personal opinion but I've never bought into "the let's go to a club or mess around and see what happens and in the heat of the moment who knows..." I always figure the heat wears off and feelings come back about that time...and some of them might not be warm fuzzy feelings
. And in this case you guys are close friends...terrible to screw that up.
Now having said all that...I'd probably hang out with them in the vanilla world for awhile before anything else. Good way to observe the relationship and get some better insight into what course of action to take.
-D