Do you mean plug the old C: drive back in, then look for it in Windows 7 msconfig Boot tab and delete it from list if it appears? Does this inactivate old C? Can he continue to use it as a data drive with old Windows 7 OS on it without reformatting?
What if plugging his old C: drive back in makes new Windows 7 unbootable? Should he boot into Partition Wizard and mark it inactive to achieve same thing? Somehow move data files off and format before replacing them?
I need to get some sleep as I did this recently and can't remember how it went.