This scenario happened to me just this weekend! I was employed to shoot my sisters wedding on the weekend. A few weeks prior, she notified me that there would be the brother of a friend of hers coming along to the wedding, and that he was in uni and wanted to get shots for practice. At that point, she was under the impression that he thought he could make a bit of money out of the event (he was asking her 'what kind of shots' she would like), so she spoke to him and kindly set the record straight (that he was to NOT get in the way of the event or interrupt any of my shots).
So here I am, on the boat with my sister, arriving to the ceremony, and what is this I see up on the hill? A guy with a 5D mkii and 70-200 f2.8L IS mkii hangning off his neck, snapping away and running about all over the place!
Long story short, he wasn't a young uni student, and by all accounts he was assumed to be the paid photographer at the event before I arrived! He messed up several of my shots, and luckily my partner had a word with him when I went off after the ceremony to get family shots, saving me the wasted time doing so.
Now I've not come on here to have a rant and rave, but if anybody else finds themselves in this situation I strongly urge you to arrange speaking to the 'keen amateur' well before the wedding, personally. I got 95% of the shots I set out to get, however there are others that are unusable (people getting shots done with the other 'tog in the mob of people after the ceremony), other guy getting in the way of family group candids etc.
So I learnt another valuable lesson, at least