I will agree with everyone else on this situation. Stay out of it. It's not your place to make swingers out of family or friends.
Now, I just have to address this statement because it REALLY bugs me. I agree that swinging has brought a new level of fun and closeness in our marriage and probably in your marriage as well; Andy and Katie's marriage might not survive. They may be very happy just the way they are. Since you are not married to either of them, you have no idea what their marriage is REALLY like. It could magnify their marital faults and maybe that would be enough to break the camels back, if you know what I mean. I'm sure it's been discussed in here that most marriages can't handle the emotions and all that swinging entails. If they don't come to you first, I'd leave well enough alone.
Maybe swinging would ruin this "fine" marriage and maybe it wouldn't. But, like it's been said before, it's not your place to introduce swinging into their marriage. If it ain't broke, please, don't try to fix it.