Wait a second - is it your old friend or his wife you have known all your life? There's a lack of information from the OP.
Forgetting morality for a moment - make a list of the worst-case scenario. Let's say something happens between you and this woman you've kind of put on a pedestal. What's the worst that could happen, what is likely to happen (as you know your old friend better than anyone else here) - and could you live with yourself? Could all parties?
I would say, if there's really something between you and your friends' wife, then surely it would wait until they divorced. Of course, if no divorce is ever on the cards, then...that gives you your answer.
In practice, it's simple - everyone will do what they really want to do, and retrospectively excuse their behaviour on some notion of morality.
I would question how much of a friend you really are, though, if you follow through on something that has little substance. If the marriage is dying, and there is a genuine connection there, that's possibly another thing...but it doesn't sound like it.