I've actually been in this situation a couple of times. My wife and I talk about how sometimes a guy just can't recognize a girl with the "crazy eye" even though every single person they know and love can see it a mile away. It amazes me every time.
My advice is this. If the woman truly has the "crazy eye" then you've already lost your friend. Things will never be the same and 95% of the time will only get worse. You'll either see his empty husk a couple times a year or lose touch completely. And all he'll ever do after a couple years of marriage is complain about how horrible his wife is and constantly be worried about getting home or getting in trouble. You've already lost him so you may as well take the chance of trying to save him before it's too late.
I was the best man at a friend's wedding. I tried talking to him a couple times but in the end we have a now or never discussion in the parking lot before walking into the wedding hall. I basically told him I love you bro but you're making the worst mistake of your life. If you want to go through with this I'll stand beside you and support you and I'll never mention this again. But before it's too late I can't in good conscience stand next to you and allow you to do this without saying my piece. I didn't call her any names or degrade her, but I pointed out the craziest of the things she had done, how she treated his friends and family, and also pointed out the fact that he was miserable with her. Then I offered to have my wife drive him right form that spot to his house to pack and then to the airport where we would fly him back to our house and he could hang with us for a few weeks while the storm blew over. I would go inside, let everyone down gently, and take the brunt of the storm. My wife and I begged him to get in the car and run.
He thanked me, told me I was right about everything I said, but then said he made a commitment to her and his family and he couldn't let everyone down. He made a final decision and he thanked me for my honestly, we hugged and walked inside to marry the woman of his nightmares.
They were separated before 30 days were up. His father in law told him she was cheating on him with an old boyfriend a couple weeks in. She cast him off like a piece of garbage. A couple months later she was living with another woman.. He was pretty crushed, for a long time. However, we remained great friends.
Many years later he found the RIGHT woman and is happily married, has a great family. We don't live near each other anymore but still get together whenever we can and talk regularly, still the best of friends.
Take the chance, be honest. Don't call her names and be emotional, point out some of the facts. Tell your friend you'll be his friend either way but you just want what's best for him. He already knows. He is just resigned to it. But most likely he'll appreciate your honestly and concern. Of course if they stay together he'll eventually tell her and she'll hate you and may try to kill you. But that's the risk you have to take