I've seen many examples of people who have hired beginner photographers (paid or not) and received such dreadful photos that they were really upset. They can't do their wedding again and all record of it is completely lost.
It's a risk - you may end up with something, and you may end up with nothing whatsoever. The couple need to understand that this is a *real* possibility.
Well, I did state exactly this, plus I think there is a lot between 'beginner photographer' and 'non pro wedding shooter' or even 'non professional' photographer. Of course you should be able to do proper photography. But the OP stated that this was his first wedding, not his first shoot ever. I absolutely agree that somebody who does not know his way around a camera should not ever do a wedding. I really doubt that this is the case here however.