The 24-70mm VR is $1800 or there about.
A 28-70mm AF-S is around $1200 for an EX+ copy.
The 17-55 is around $1200 new.
A Tamron is around 500 for either I think.
Will you be able to use flash in the wedding? You will need to ask the Minister/Priest/Rabi to find out for sure.
If you can use a flash then an 18-70 would do fine with a good flash technique and
a better bounce card (google that to find out how to make one). If you would rather have something that looks more expensive then go here
http://www.dembflashproducts.com/
If the church is big enough to warrant a big zoom then a Pro is probably in the budget and she should go that rout. If she will let you pick one out maybe they will let you assist and give you some pointers while you are at it.
OK, now for the best advice. IF a pro isn't in the budget, then hire one yourself for a morning or afternoon to go over your equipment. What you are going to need. And then show you some techniques that you will be using and
should be Practicing before hand! You will need to explain what is going on and may have to make several phone calls to find someone who will agree to help but I don't think that it will take too long to make a connection. You might also get to assist in a wedding or two if you ask nicely. (Free to the photographer of course)
Do expect to pay for this and Do follow the advice.
HTH,
mike