Donna,
Maybe the plumber is right, and maybe the builder is right. You gotta plug that drain and fill the pan with a bucket. Let that puppy set for a day and see if the water leaks. If the pan leaks, replace the dang thing. Remember, that tile pans use either a lead or vinyl pan to collect the water. It's not the tile or the grout that holds the water down there. It's that dang liner.

Now if that dang pan isn't leaking, THEN check for other things, like around the faucet. Smear caulking around the plate and in the corners.

But remember, If the pan is leaking, then who cares? Mad Plumber

: There is water leaking under the house from the shower. I was told by the plumber that I have a leaky shower pan and must replace it. I was told by a contractor that it may be leaking simply because the tile needs re-grouting (which it does). Who do I believe. Obviously, re-grouting would be the simplest and cheapest solution, but since it's leaking under the house doesn't that mean that the shower pan must also be leaking? I'm totally confused! Help!