Thank you, everyone, for your replies and suggestion.
After some discussion, we have decided to take this route that I am going to lend him my 18-55 for the time being.
Today, at a photo fair, he tried his hands on the 20mm f1.8G and 24mm f1.8G, and he instantly fell in love with the primes. He is even thinking of going back to full manual focus as he feels that he can wield far more control of the scene.
Thank you all again.