I use SFC every few months and run it to fix any corrupted/damaged system files.
The vast majority of users NEVER
need to run SFC. He's got other problems if he needs to do that. Continually replacing a bandage on a wound that is constantly being re-opened will never heal the wound. He needs to stop whatever it is he is doing that keeps breaking his computer. Constantly running SFC is not the solution. He needs to change his computing habits, then perhaps he should look at imaging programs.
And besides, the reason SFC uses the files on the HD and not the DVD is because it is likely the DVD files have been superseded by various updates and patches.