Really, I believe there's no such thing as best. I've even shot at 400mm to reach at the back of a church hall (I'm standing in the front).
I'm a newbie shooting church functions (first holy communion, comfirmation, etc) for free . I normally bring a wide lens (12-24mm) for group shots, a 18-200mm for general shots, a 18-50mm for portraits.
haha. with ur skill, ur friend will be very lucky to have u shoot
i not too sure abt nikon lens but i think range of 17-55 should be quite adequate for most chinese wedding, and bring along a couple of primes, mebbe a 50mm and 85mm to get nice bokeh shots and provide a little more reach
Kit lens like 18-55 definitely can be used, especially when u shoot with flash. Have shot before with Canon's equivalent for a friend's wedding and they loved the pics. But it does limit ur creativity though and it's difficult to take pics with shallow depths of view (e.g. selective parts of the photo in focus). Also, the "smoothness" of the out of focus areas are not as nice as the better lenses.
For the first poster, the best lens for u could be different from another. A lot depends on the shooting style. Usually, u'll want a lens that's wide enough (28mm equivalent after the 1.5x crop factor) cos u'll need to shoot groups of people indoors. As for how tele and how wide an aperture, it'll depend on how u shoot. If u're more interactive with couple and guests and u're close to the action, u'll likely not need a long lens.
i think weddings u will need 2.8 glass for low light?
mabi a standard zoom range 17-55mm and a tele photo? 70-200?
kit lens can be used,but juz like mpenza said bokeh cannot match up to 2.8 glass. and focus hunting might cause u to lose shots.