Though I do believe healthy relationships like the one you are describing Sutekh are possible, you do understand that they are definitely not common at all, so you cant blame the forum to be skeptical. Like Eidal mentioned, are you a 100% sure the husband has no beef against this, and if he did what would your reaction be? Would you honor his opinion that you hanging around is not a very good idea, or would you be offended by the fact that his own insecurities are breaking apart your life long friendship and that you know that she would not be cool with that? Maybe you would even tell her and that would most likely cause drama between them, because obviously, you don't want to let the friendship go. But the problem could be that neither does he.
The perfect scenario is that everyone involved would be mature and completely honest with themselves and each other for there to be zero problems, and if that is the case, then that is awesome, because you truly have one in a million type of friends, which means you should totally go to switzerland and enjoy the free and hospitable accommodations, otherwise i'd think about it a bit more.
In the future this will also involve the person you may want to settle with later on. Is this person also gonna have to be cool with the whole thing, or will you just end up saying no to someone that doesn't understand this?
It's always Very difficult to be very close friends with a married person of the opposite sex, specially if you dated her previously. Impossible? No. But you have to agree that it is highly unlikely.