My honest advice is to advise them to hire a professional photographer. Weddings are too important to leave to chance.

That said, you need to get some off camera flash, diffusers and reflectors. Maybe you can rent if buying isn't an option.

Another thing to consider is whether or not flash will even be allowed during the ceremony. It is a bigger issue inside churches and such, but you need to check with the person conducting the ceremony to see if they permit flash.

Check out as many of Joe McNally's outdoor shots as you can on his blog. He's good about teaching the way it was done.

Good luck!