A big question about how to approach this I'd say is ----- Are you thinking about shooting more weddings after this one? If the answer is yes, then within reason go for broke on it ---rent a backup body and a 70-200 lens, read, research, watch tutorial videos...if you can, find some other events and get access to them, shoot them, learn from the experience.
If the answer is no, and this is just a one time deal to help a friend - then let them know this is what I have to offer and do your best! Enjoy the open bar, have fun shooting!
Also, 16-35 vs 24-70 --- each have their own uses....bring them both!!!!...for the ceremony, set your gear bag in an easy to access spot. That 16-35 will get you great big shots of the overall happenings...and if you can get close to the B&G, some very dramatic shots as well. That's touch and go though, you don't want to be so close that your an annoyance to guests, B&G or the venue itself. Regarding switching lenses during the ceremony...depends on the ceremony! Some are very Quick...10-15 minutes...but some can take up to an hour, and ---for those hour long jobs...there's a lot of nothing going on for long periods of time (readings, prayers and blessings, kneel and be silent while the organ plays...so you have to be ready for the action when it happens...so mange that downtime well!!!!