What should I do or say? My buddy is making a huge mistake?

Maybe you can ask him why 2 weeks? Is there something magic about that number? Maybe it isn't saying "no" so much as it is encouraging him to WAIT. Where is he getting married....most churches want pre-marital counseling. Perhaps you could encourage that and maybe you will feel better about things if that goes well. And if they go through pre-marital counseling perhaps they will discover they may or may not be made for each other. My personal conviction is not to live with the person but spend time getting to know them before getting married.