The point is who cares? I mean if he's paying and not doing anything with it, that means he's not using your bandwidth, I've had customers purchase all the time and never use a single MB, I mean you can't force them to use it, can you?
Well, i have no problem, that's for sure. Just wondering myself. Spending money for doing nothing with it !
From pas experience I've seen such thing happen yet there is file inside his account. I guess your client edit his host file and point it to your server instead so that he can work out on something invisible from anyone. Unfortunately my client use it to host his rapidleech script.