hi mate to be honest i think your getting worried over nothing , however if hes used so much adhesive it may mean hes had to bed out quite alot .i.e put more adhesive behind the tiles because the walls are out,this obviously means it needs more time to dry...
however grouting the same day is not uncommon, also if hes done 15m2 in a day and it looks ok then this guys obviously got speed , speed comes with practice so he may have been tiling some time and know what hes doing (he mite of thought it was fine to grout )
shaun.....