before you go into all of that though is he sure the smell is the tank its self and not something IN the tank?
Seems like if it were the tank its self, he would have never been able to clean the smell away to begin with, and that the water changes wouldnt make much of a difference.
Possibly something that he put in as hardscape, or the substrate causing the smell? I have had soil tanks not work out for me and smell horribly, I have also had wood turn water pretty stagnant and stink like rotten eggs.
Just something to look into before the tank gets ripped apart