If it were me, going into it with what you have now and operating on the premise of single-camera wedding shoot, I'd (rent or buy) a 16-35/4IS, along with the 100 Macro already suggested. Imagine for a moment that the day before the wedding, your 24-70 dies: you're not going to survive a wedding with a 70-200. However, if you go into it with a 16-35 and a 24-70, either ONE could die and you're still safe. I'd also rent a 5D3 so you have something semi-comparable that you could fall back on if your 5DS should go on vacation.

After that, I'd prioritize "getting the shots" over "having the best image quality". A shot with great IQ but taken after the cake smash is not worth nearly as much in memories as a shot that's a little soft but captures the emotion of the day. Honestly, 5DS with 16-35/4IS and 100 Macro (f/2.8 with IS, great for rings, great IQ, not so slow to focus that you're screwed at a wedding) might be all that I'd take in, leaving the 24-70, 70-200, and 5D3 in the trunk (out of sight) to be grabbed if something happens on the spot. As a freebie no-budget wedding, if you (the friend, amongst lots of other friends) were to jump and say "wait a minute, my camera just died, I need to grab my spare", folks will push the pause button and enjoy their company while you make the mad dash to fetch, and they'll keep an eye on whatever gear you park in sight of them while you do it. Rolling with a distinct setup, 16-35 for stuff that's wide (knowing you can comfortably crop to the equivalent of ~60mm while still having 22mp at your disposal) and 100/2.8 IS (which you can crop to 160mm equivalent, if not more) means your lens selection decisions are simplified IMHO. Both of those lenses are killer good on my 5DsR; the 16-35/4IS has completely displaced my 24-70/2.8 (both old model Canon and new) as my "first alarm" lens.

Now, if I had wedding experience under my belt, an assistant, and preferably a second 5DsR (something about it just blows away my 1Dxes), I'd roll with two cameras and an assortment of primes: 14/2.8, 35/1.4, 50/1.2, 85/1.2, 100/2.8 Macro, 135/2, and 200/2, with 1-2 Profoto B2s on stands (B1s if it's high noon). But that's how I roll...