This forum is only interested in the legallity of your request.
The OEM license allows you to upgrade/update any hardware inside your machine with the exception of the motherboard.
If your motherboard fails and you are sent a different unit under warranty then this is fine, however if your motherboard fails and you simply buy a new one then again you are no longer license legal.
What happens if his mobo packs up and he built his own system? wouldn't that make him the system builder and allow him to change his own mobo and reinstall OEM Vista on the new mobo.