What the others have said already. Keep out of it but be there if they need you.
As for suggesting swinging, this is the last thingthey should try if their marriage is not 100% solid. Swinging will definately not fix a bad marriage but it can, in some cases, enhance an already brilliant one. You need a very solid foundation for this game and if that isn't there, swinging will only make things much more worse than it is now, possibly resulting in no marriage at all.
The only advice I would ever consider giving friends in this situation, is to talk, talk and talk again, but make sure it is to each other and not to other people!
Good luck Adi, not a nice position to be in.
Vanilla? I thought that was ice-cream....have I missed something here? Learned people, please put me right on this before I commit an awful gaffe next time I go to the cinema....
If advice is sought by people not known personally to me then I would listen, with fortune to both sides, at least and help them to develope a strategy to come to their own conclusions.
I have been known involve myself, with best intention, in freinds problems I admit. Possibly making things more difficult and awkward than otherwise they may of been. To see a friend in distress though is a heart wrenching thing to see and sometimes just sometimes its not the head that thinks, and the heart can make such mistakes. To err is human, as they say, thank god we are human. Always something to learn.
Swinging is the worst thing you can do if your relationship is in trouble.
If your relationship is strong and the trust is good, but prehaps the sex side is becoming a little stale, maybe that would work, although we never did it for that reason.