Technically, you don't know.
It might be a driver problem. All kernel mode drivers are well able to mess around with the PFN list, causing stop 0x4e. A buggy driver (they upgraded the hardware) is all it takes. Double free anyone?
It might be a hardware problem, that's one of the three. It may be the RAM, it may be something more subtle. May be a thermic problem.
Maybe they caught a rootkit. These may run in kernel mode as well, and generally hook several system calls.
Also, he does not need memcheck, as Windows 7 can do that just fine. They simply have a localized version of it. Have them press Windows+R, type in MdSched and hit Okay. Windows 7's start menu is like WotLK. People don't know where the tools are, anymore.