They obviously never intended to hire a pro, I would guess for economic reasons. As long as you manage their expectations, explaining that you are by no means a competent photographer, if they can't get anyone else, do it.
Some years ago, before camera phones were available, I attended a wedding and the couple had disposable film cameras placed on every table for the guests to shoot pictures. As crappy as those cameras were, I took a few shots that were better than what the pro took. Recommend to the couple that they encourage the attendees take their own photographs using whatever cameras they brought with them. This way they'll have more photos than what you take. They can create a Facebook page or choose some photo upload site where everyone should upload their photos.
Although I ordinarily do not recommend that people use auto, in this case, because you're inexperienced, you should set it Auto. If you want a little bit of flexibility, use Programmed Auto.
Do not use the camera's flash. Set the ISO high enough that you can shoot without it. The pictures will be noisy, but they won't have the harsh, amateur look of direct flash. If you're shooting the wedding ceremony, get a tripod, because they'll probably be little light.
If you're young and can hold a camera very steady, you can shoot as slow as 1/30th of a second. If you can't, don't shoot slower than 1/60th of a second. If you have only the "kit" lens, it's a slow lens, so you're probably going to have difficulty even being able to shoot at those slow speeds. You'll be shooting "wide open", but there's an advantage to that: backgrounds will be out of focus. Because of that, when you shoot, make sure everyone is in the same plane. If someone is standing behind someone else, they're going to be out of focus. Also remember that when shooting at slow speeds, you can (hopefully) stop your own movement, but you can't stop the subjects from moving. This is what amateurs don't realize about the Vibration Reduction lenses: they help stabilize only your movement.
Do not attempt to take portraits. You won't be able to pose people properly and you don't have the lighting. Instead, do "event" photography and take candids. Think of it as telling the story of the wedding.
Make sure you have enough memory cards to shoot a lot. Go buy more if you have to. By the way, don't just shoot JPEG. Set the camera to shoot both RAW and JPEG. That way, they can turn the RAW images over to someone who knows how to use Photoshop where they can be vastly improved, especially as to color balance, which you'll probably get wrong. They can also "airbrush" the bride to make her look more like she thinks she looks.
That will take up much more room on the memory cards, but it's worth it.
There's one more thing you might consider and that's renting a better lens. Renting a faster lens (like a 2.8 zoom) will gain you 1 to 3 stops, which can make a big difference in low light conditions. I recommend renting the 70-200 2.8. It's heavy, but you'll get some beautiful images, especially great closeups, which the camera phone users and point and shoot users won't get. Just don't break it because it's a $2400 lens. (In fact if the rental company offers insurance, buy it.)
Whether you rent the lens or not, before the wedding, practice shooting indoors. Get really comfortable with the camera. Don't just shoot - upload the images and look at them full size to see where the problems are.
Good luck.
