Before anyone starts freaking you out about how you should have two camera bodies, spare lenses, spare batteries, how you should go the day before and work out all your compositions, settings, lighting, if you can use flash in a church etc etc.
Find out what their expectations of you are. Have they seen your photos before?
I have assisted at a few proper weddings, and shot my uncles wedding, and what I can say is, all weddings are different, and as Douglas Adams would say "Don't Panic".
My uncle's wedding was on the rooftop of a building overlooking the beach with a celebrant and 8 other people.
This wedding may not be in a church, with hundreds of people yada yada.
When people are spending tens of thousands of dollars on weddings, they rarely ask their friend to do the snaps for free. You just know there is going to be disaster, and the bride and groom know it.
My experience has been that most couples who hire cheap photographers or enlist friends to assist, is that they don't really care too much about the quality, and are usually surprised as to how good your images come out in general.
They may have an album, but they usualy only frame one or two shots, and if you can't take one or two nice shots that are frame worthy, then you probably should turn down the offer anyways.
Most people these days are used to candid P&S shots. When they see a shot with DoF, and great lighting they literally wet their pants.
My best advice is to get a speedlite / external flash, and make sure you have your camera batteries fully charged and some spares for the flash.
You probably have 2 kit lenses, a wide angle and telephoto.
You'll be fine.
The last advice I can give you as the dedicated photographer, regardless of the size of the wedding. Don't get involved (personally) in the parts you need to take photos of.
For example if it's your best mate getting married, and they have their first kiss, take a photo of it, not clap along with the participants.
Don't listen to the speeches, take photos. Don't laugh along to the jokes, take photos. Don't socialise with the guests, take photos.
You need to remove yourself as a participant, and become more of an observer.
You may "miss" a lot of the day, but you're the resident photographer, so your presence is better served as capturing the day, not being a part of it.
That's my advice.
Get a really accurate impression of what they want, and let them know what they're in for.
If they want professional photos, then recommend they hire professional. Because a Rebel XT, no external flash and kit lenses will more than likely not make professional photos.
And with the money you spent on purchasing appropriate level gear, they could have hired a professional and saved you the stress.
Hope that didn't come off too sharp or cynical.