He could get a practice kit, or he could look at the possibility of mesh heads for his existing kit, which are much quieter. Or just get some rubber practice pads.
My bloke never plays the kit at home. Similar reasons - we have nice neighbours and no desire to upset them. However, he practices at home regularly. You see, he's a 'tapper'. He's always going around tapping his beats and rhythms, with or without sticks. He taps cushions, worktops and his own legs.
It's bloody annoying and it can't beat the real thing of course, but it's a workable compromise.