Did he use the 'new' mobo and cpu to replace and IDENTICAL mobo and cpu?
If not, then he will have to either install a new harddrive and new OS and drivers, or he will have to reformat his current drive and reinstall the OS etc.
If the old hard drive is big enough he can reinstall the OS (if it is windows) on that drive, and the old os will be renamed: He will still have to reinstall all his programs.