I understand what Kudzu is saying, but at the same time, maybe it isn't up to the OP to decide what is best for his buddy.
Apparently he has let his friend know what problems he has with the marriage. Maybe his friend is fine with the relationship. How does the OP know WTF is going on in their relationship?
If the OP knows something that the friend does not know, I think he should be honest with the friend BEFORE the wedding takes place. In fact, the sooner the better.
If the OP just doesn't like the woman, that is his own fault. He isn't marrying her. If his friend is fine being a cuckold, let him be a cuckold.
And for heaven's sake... with regard to the toast, just say something nice about the groom and be done with it. Don't even mention the woman at all. No one cares about the best man, anyway.
The best man is there to support the groom. He isn't his personal relationship adviser.