After reading your comments I've now been considering is talking to him into hiring a photographer, and then I'll just bring my 6d, and pancake lens just for some candid photos to give to them.
If he insists on me, I'll then tell him to at least hire someone for the ceremony and I'll shoot the reception.
If he really insists on not hiring a pro, then screw it, I'll just take my stuff and he'll get what he gets!
I'd much rather see them with quality photos than meh photos, from their friend who just does it as a hobby.
Sounds like you're thinking clearly about it. I'll add a few more items in case you end up shooting. I'm an amateur who's done a few wedding gigs as a second shooter , so this is from someone closer to your level. (apologies if others have covered this ground, I haven't carefully read the entire thread)
* Talk to the bride, in person, and make sure she understands the expectations. The fact that your friend says it's OK is not good enough. Her needs/wants/expectations may be different from his.
* Weddings move fast, so simplify as much as you can. Don't expect to have lots of time to change gear.
* Better to know a few items of gear well, than take a lot of stuff and lose track. Take one good low/medium zoom, and one long zoom. No other lenses are needed.
* using bounce flash in the reception (or at the ceremony, with the agreement of the couple and officiant) is not that hard. You can spend 30 minutes with a couple friends in a dimly lit room to figure out the settings you need. (hint: use manual with flash) Don't get creative with flash until you know what you're doing.
* Get the "script" of the wedding in advance. You'll need to anticipate the action to be in position.
* For the reception, couple pictures, and other photos for which you have time, don't be afraid to take a little time to set up a shot, or do a little directing of the action. Yes, it's their wedding, but they'll be happy to let you guide them to a better shot, for example to get a better background, nicer lighting, etc.
* If it's a big "family and friends" wedding, try to get photos of everyone, especially older relatives and close friends. No one knows how much time great-Aunt Helen has left, and they will appreciate photos of her dressed well and with a big smile.
* Take a few cute photos of kids.
* If the wedding is on Sunday, see if there's a local wedding on Saturday, then offer the pro photographer to be his/her free assistant for that gig. Seeing behind the scenes just once can make a difference.
* Take your time with the group photos. Use a tripod, live view and check your depth-of-field charts (in advance) to make sure you do your best. Don't use flash here unless you can bounce off a high ceiling or back wall. Be willing to turn the flash off and make do if needed.
When in doubt, turn off the flash, set to P, and go for composition rather than technique. Your 6D will do well in low light compare to all the P&S in the crowd.