That sucks ell
This is one of those nightmare scenarios that I know I will run into sooner or later after I get started. It will be some obscure problem and it will make me look like a moron.
If it's the windows update you should disable them along with security center so he doesn't update his computer.
Also after you install windows make sure you get all the updates and keep it running. Get whatever else he installs at home like printers, programs (everything) and install it for him and see of that breaks it. Don't charge him. At this point his trust in you is gone and it's up to you to reinstate that trust. That is if you want to keep him as a customer.
My guess is that it's a software problem. He obviously installs something at home or does something to cause the problem you just have to find what it is, or remove yourself as a culprit in his eyes.
If installing all the programs updates etc doesn't break it then bring it over and show him that it runs and the problems couldn't be reproduced in the shop. You may lose him as a customer anyway but at least try, you may learn something. After all you will run into problems like these again, so you better get used to dealing with unsolvable situations or learn to solve hard problems.
Sometimes it's just the fact that you try and stick with the customer that will regain their trust. Don't leave them out to hang.
What program did you use to get those minidumps like that?
oh, I just pasted it into excel so it was easier to read, then made a jpeg of it. Just got it back, and it booted fine then while I was reading the logs it BSOD, win32k.sys crashed.