Actually, I'm just an old goof that shot weddings for the first five years with 1 camera, a Nikon f, 2 lenses (50 & 135) and a Honeywell Strobonar 892 flash. Never gave equipment failure a thought. Dropped the Nikon from eye level once, picked it up started taking pictures. Now I carry three Canons around my neck and pray that the one I'm using for the Bride walking down the aisle doesn't decide to err99. So I definitely understand the concern about using one camera at a wedding these days.
If the guy is using a 580EX he can probably still stop down to f8, which is probably the sweet spot for that lens, thus pretty sharp results. He should be able to bounce off a ceiling or wall or umbrella at f8 at about iso 800. Use a bit of noiseware and produce some decent images . Granted, this is not optimal, but at his price point it is reasonable. IF, he knows what he is doing. The fact that he has a second shooter at this price point is amazing and a possible bonus to clients looking in this price range.
I'm not trying to get into a pissing match with anyone, I'm just trying to point out that if the guy is a people person. (the most important part of wedding photography) has good referrals and portfolio, covers his butt with a second shooter, then maybe he isn't so bad after all.