He should replace it under the customary one year builder's warrenty.
I have had this show up later. I simply dug it out with a grout saw
(handy tool!!) and regrouted with silicon caulk. The base of the
shower had settled and the grout had pulled away. 1/2 hour job, at
most. Once the house "settles," this problem should go away. I wanted
the resiliency of the silicon caulk to avoid the problem of "mortar"
type grout breaking up/cracking in this case.

When I did some tile work in a bath a couple of years ago, I ran a
small bead of silicon caulk around the edge of the tub before I set
the tile, then grouted in the usual manner. Any "flex" seal problems
got taken care of by the silicon caulk.

I'm not sure if this is the accepted way to deal with it, but sealed
up nicely in both cases.