First, you should talk with your friend about this (the marriage) and express why you think marriage is a bad idea. Of course be very careful and tactful about what you say, but if you are the best man, he apparently thinks you two are close, so it is your duty as friend to talk to him about it. Do not be too pointed with your talking and do not express any firm beliefs. Just say things like, "Isn't she in debt? Are you okay with that? You don't think she would ever use you for money?" You want to get the wheels turning in his head without coming across as a jerk.
Second, go to the wedding and be best man. If it blows up two years from now, oh well. It is his life and his decisions. I have actually seen this several times already. One of our good neighbor friends growing up married her long time on-again off-again boyfriend against the wishes of pretty much everyone who knew them. She then had his child, he continued to be a deadbeat loser, and she divorced him within 18 months I believe. Another was a friend of friend. His wife was hated by pretty much everyone who came into contact with her except him. She treated him like garbage, stole from him, spent all of "their" money, cheated on him, cussed him out in front of everyone at his work place, etc. They divorced within a year or so also. Both of the above (the neighbor friend and the friend of a friend) are now happily married to other people, but no amount of talking would have swayed them with their first marriages. It might even be better for you to not say much at all. It's your call... but I would still be best man for the guy regardless.