And a second body isn't that necessary since this will be his first wedding. He just needs at least 8GB of memory with him (well that's more then enough for me)
I completely disagree about the second body. He is THE photographer, what if his camera fails, what if he has a 50 mounted on one body and sees a shot that needs a different focal length? Once someone has shot for a long time they will have an idea what equipment they need ready. This photog needs everything ready at all times as they dont' yet have that experience.
And bring craploads of cards. It is easy to shoot 20 gigs at a wedding (especially if you are learning and just shooting everything and sorting it out later). If you think this will be a one time thing, buy a few cards from your local big box store and just leave them in their packages in your bag. If you need them, they can be opened, if not, return them.
Personally shooting crop, I would have one body with a 24-70/24-105/17-55 mounted to catch most shots, the other would have a 50/70-200/10-22 depending on the current situation. Obviously these are canon lenses, but nikon will have something similar. For some shots recently I have been using strobes with a remote trigger on each body, but they definitely have a distinct look to them and you lose a lot of the ambiance (but can catch shots that are impossible any other way (ie group dance shots)). I would recommend at the very least a 580 (equivalent Nikon SB900?) with an omnibounce or lightsphere on each body (omnibounce prefered, lightsphere will likely get knocked off if you are shooting two bodies). Off camera with pocket wizards can be better, but you will already be so overwhelmed with everything that you will likely have more luck with just two self contained bodies.
Make sure you get a good tripod.
This will be expensive for you to rent all of this stuff, but required if you want a chance at professional level shots. The last wedding I shot with a friend, the rental was ~250 for lighting only (PW's, Metz, light stands), we already had all the glass,bodies,small flash guns,strobes we needed. For that wedding we used (note, not all of this is required, but we had it and used it and got keepers from everything).
Bodies
20D,50D,Xsi(primarily used for time-lapse shots/backup body)
Glass
24-105, 70-200 2.8 IS, 100 macro, 30 1.4, 50 1.4x2, 85 1.8, 10-22, EF 2x
Light
AB800, AB1600, 580II, 430, Metz CL4 most with light modifiers (omnibounce, lightsphere, soft box, umbrella) and stands are required.
Tripods
055Pro Carbon, 190CLB aluminum
Storage
2x8gb,4x4gb,2x2gb,2x1gb,100gb Hyperdrive (all cards were copied to hyperdrive as a backup)
Batteries
3 batteries/camera (only 2 used), 3 sets of eneloops/flash (changed once part way through to keep flash speed up)
Misc
Stroboframe ProRL, L358 lightmeter, 4xPWII, 3xAB RR1, 2xOC-E2, EU2000i, Reflectors (Large and small), Camera bags, Background stands and paper (weather turned bad needed to do wedding portraits in an ugly hall), sandbags
Like I said, you can shoot with less, but some things I personally consider necessary when it is your job to shoot the wedding (ie multiple bodies, multiple batteries, multiple flashes, tons of storage space).
When shooting weddings for fun (ie. not THE photographer) I normally drop to one body with a 580II (w modifier), bag of lenses and sometimes the stroboframe (although that gets a lot of looks and comments so I don't generally pull it out, it does help take some great pictures though).
Good luck.
