Hi SA Spurs1, Wish I could give you the answer you want to hear but cannot. WMP is part of the core operating system and can only be reinstalled by reinstalling Vista OS. The WMP 12 that will be for Win7 won't be compatible with Vista either. There are many 3rd party players that are far superior to WMP and I would suggest your cousin try a few if that is all that is wrong with his Vista at this point. My WMP11 corrupted 2 weeks after install of OS. I use WinAmp,Sonique,Foobar,RealPlayer and CoolEdit to name a few. Here's a link if you want to further explore repairing WMP.
http://zachd.com/pss/pss.html
Best of luck to you.