Like you've mentioned and like most people here say, try to be just a guest with a camera, and let someone else work
However, I was in your shooes a year ago. A good friend of mine asked the same question (they hired a professional for the ceremony) and wanted to have me take the rest. I do paid jobs (portraits and stuff), but if I do something wrong (or the pictures turn out not as good as I want them, I always have the option of taking them again. Not so with a wedding...
Forget everything your friend says about you being a guest and that you should enjoy yourself. You will be so consumed in doing a good job for him that you will not hear a word in the speeches
I was so scared that I would not get this right, that when I gave them the pictures they received 1400 high resolution jpg's (all was shot in RAW), and I told them that they could pick 10 pictures that I would work on in lightroom and photoshop. The good thing is that according to them, my pictures were much better then the professional (BTW. if they do get a professional after all, stay away from their working path...).
If you do this, the best advice I can offer is to follow the bride! She is your real constituent
In addition to the required shots mentioned by others I would try make some context shots, like the food, flowers, the cake and such. People tend cherish these details.
Different countries have different customs. In Norway it is not uncommen for the bride and groom to hold a speech for each other. Some of my best shots that evening was them looking at each other (and the following laughter, crying ect) when they spoke to each other.
As for lighting I agree with the previous comments. Stay away from it, but there is one instance where you cannot avoid it - the waltz. I if I were you would make sure that the flash is in ETTL mode, and switch to the green button to let the camera do the whole job for you. That is a picture you will not feel comfortable screwing up, lol.
Enclosing a shot I took of the food that night.
PS. I bought a 85 after that wedding as I saw that my 135 was too long (to many people who move between you and the subject) for many of the guest shots.