I agree that encouragement can help people do apparently miraculous things sometimes. But inappropriate encouragement can also lead people to do things they're not prepared for. The trick is encourage our friends to challenge themselves without sending them off to hurt themselves.

As an addendum to Denise's idea about training, is it possible to just take your friend out for a longish ride or two? If he's untrained, just a 30-40-mile ride may be enough to show him the difficulty of what he's planning to get himself into. If he gets to mile 30 or 40 and is tapped out, you can maybe gently bring up the idea of having to do that same thing times 2 (and then some!) for 7 days in a row. OTOH, if he goes the distance (and can do it again the next day) and still feels excited about doing the AIDS ride, maybe he does have a shot.