So I took most of your advice and rented the 24-70 and 70-200. The gear should be here on Thursday. Wedding is on Saturday.
Also, I'm not just showing up as a guest with a camera. He specifically asked me if I could help. There's also some times when the other photographer won't be able to be in 2 places at once. I'm definitely not pro, or claiming to be. He can't afford to hire a real wedding photographer, so I'm doing what I can. I'm a fan of available light photographs, so we're hoping I can get some good shots that wouldn't be possible with her crop body, slow lenses, and flash.
I'll report back after the wedding with samples for you guys to critique lol. Thanks for the advice!
5D III + 24-70 II and 70-200 f2.8 IS II = "SUPER COMBO"
If the locations are well lit, the gear you rented will do just fine. I'm not sure you will need flash, but it's better to have one on hand.
Since you are Nikon shooter in the past, make sure you give yourself plenty of times to play with 5D III - get to know all settings and button locations.
Stay away from "Green" & "P" mode...have fun and post some pics