Whatever you do, don't charge them.
Have them sign a release stating that they know you are not a professional and that the results may not be as good as desired.
Always take your camera if you have a chance to visit the place ahead of the wedding. This has saved my butt in the few weddings I took for friends (and no I am not a professional and refuse to be paid for weddings, and refuse in general to do weddings now).
Read, read read. Just because you're excited and they liked your shots from a different wedding doesn't mean they're going to like the pictures of their wedding. Learning about lighting is GREAT...but it is not everything. Take into account the fact that a white dress against the dark clothes of the wedding team can make things a bear. Take into account how many people will be in the wedding...getting everyone into the shot and having it turn out so that every person is seen sharply can be a task. Angles angles angles...when you are THE photographer you have to be creative, innovative, and on your game. You have to be assertive but friendly and if they don't like the way you're doing something you have to be flexible. You can't have an attitude that tells them that you know everything and they know nothing. While the customer is not always right in photography, they have to feel like they are, so if they ask you to do something specific, you have to be prepared to be as creative as possible in making that particular situation work.
I have shot about 5 weddings now and all of my "customers" were non-paying and WELL informed about my lack of knowledge in that area. They were plenty happy with my pictures which was pretty shocking...but again, I never led them to believe that I was any better than I am. If it's an outdoor wedding it may be easier since some churches indoors are absolute pains. If it's indoors, you need to have an external flash at the very least to get anything close to decent shots. If it's outdoors, you have to work with the exposure, especially if the sun is brightly shining and there are shadows. Speaking of shadows, watch your shadows. You don't want the bride or anyone else there looking bigger than they are...that's for sure.
Have fun and learn lots...it will be tiring so make sure you have plenty to drink, extra camera batteries and batteries for a flash if you need it...backup equipment...and possibly, a backup person who is willing to shoot with you so the bride has more pictures to choose from. and definitely take your camera with you, no matter where you go, but especially since you're going to the wedding site. Take note of the natural shadows happening, the possible lighting issues, the setting in general. Have your friend pose even with a white sheet or white shirt on so you can an idea of what kind of settings you'll be needing to work with. Play with angles too. Good luck.