I don't typically do this but I could use some advice.

man - what a tough thing... if he's really a good friend, and he asks you to stand up for him, you do it - whether it works out in the long run or not, he will remember that you said no.. so be a friend and say yes. But if/when he asks, tell him the score as you see it - something like "you're my friend and I will do this for you because of that, BUT be aware that, to me, and as far as I can tell, I think you're making a mistake.." - go as deep as you want from here, tell him you want what is best for him etc etc... be clear, be HONEST - be careful too. Don't get into calling her names and all that.. keep it simple and straight forward. Let him know you are there for him - from day one until......
Support your friend and be there for him... from the sound of it, he's going to need you sometime down the road.