My good friend just moved back into town with a SS conversion he picked up in Madison, WI. He doesn't know much about bike mechanics, but I know even less. Yesterday he wrecked the bike because the chain slipped off the cogs, which he told me happened alot when he pushed hard on the pedals (chain slip, not wrecking :p ). Should this be happening at all? Did he pick up a crappy conversion or what? Can he just do a simple adjustment and be fine thereafter, or should he take the bike to our trusty LBS? Neither of us have much experience with Fixies, or Single Speeds, so we really have no basis for knowing how simple, common, or odd this might be. TIA

could be that the chain is too loose, or that the cog is very worn, or that the chainline is off. if neither of you know what you're doing, take it to a bike shop.

Chainline not correct is a major cause of chain coming off on a fixed gear. Chain should be tight, but not binding. You have to work at it to get it right. Try the following link for more information.
http://www.sheldonbrown.com/fixed/index.html

We had taken it to the LBS a few weeks ago, where my friend mentioned the chain slip. The mechanic simply pulled the wheel back in the dropouts to adjust the chain tension, and we assumed that was all that was needed. The mechanic didn't mention anything else. To the untrained eye, the cog wear seems normal (and the chain seems just as worn), and the chainline seems good. How few mm's could the chain line be off for it to cause slippage?

if your chainline is off by more than 2-3mm, you might want to play with it a bit.

Hey........is the rear axle slipping in the dropouts/fork ends? Could be pulling the wheel forwards and gradually working the chain off. Check it out. Maybe make sure he's got proper track nuts. Please, no jokes. hehe

Thanks for the help, guys... before, I was dumb... now I am merely stupid!

Is this a bolt on wheel or a quickrelease? I've seen many QR SS rear wheels, and all the ones I've rode tend to slip in the dropouts. You can remedy this by installing a bolt-on skewer ($15 for a set.)

Well? What was the problem? Otherwise we have no way of knowing exacly HOW stupid ya are :)