I do not believe that you can install Windows 7 directly over Vista.
To install Windows 7 on a Vista machine requires that you format the hard drive and start over with windows 7 and then only if the machine has enough resources to satisfy the requirements of Windows 7.
My son who resinstalls the OS on his PC for fun says you should format. You can upgrade over Vista but its not recommended.
You can upgrade Vista to 7 in-place, if you have the upgrade edition purchased. It’s only the non-upgrade edition that requires a clean install, format, etc.
If they sold him the computer with the credit for the upgrade, then he can most certainly upgrade in place. If he doesn’t have the CD, then he’ll have to go to the MS website, find the Download Center, and use his Windows Product Key (from the W7 Upgrade Card) to download it. Then, he should be able to just run it and upgrade it from the file on the hard drive.
I upgraded over Vista using the upgrade designed for windows vista. The process took an hour or more. Worked just fine. System has worked fine for 18 months now.
According to the Microsoft website, you can upgrade from Vista to Win 7, but the options are somewhat limited (from the Win 7 upgrade website FAQ):
“If you’re running this edition of Windows Vista you can upgrade to this edition of Windows 7”
Windows Vista Home Basic-> Windows 7 Home Premium, Windows 7 Ultimate
Windows Vista Home Premium-> Windows 7 Home Premium, Windows 7 Ultimate
Windows Vista Business-> Windows 7 Professional, Windows 7 Ultimate
Windows Vista Ultimate-> Windows 7 Ultimate