Just another thought, was the plaster primed before tiling? I have seen it many times where the bond has been broken between the adhesive and the wall, this causes the grout to crack and come loose and of course lets water in.
Either way, the installation was not done right. It really is as simple as that. How you proceed from here is really down to you but you are well with in your rights to demand that he puts it right at his cost. By putting it right I mean ripping out, letting the wall dry correctly, tanking then retiling (solid bed not dot and dab) and then correctly sealing the enclosure.
He is of course not going to want to do any of this and will try every quick fix under the sun in the hope that it sorts it but it wont Im afraid.