He will either use a Nandroid backup from stock or
[ODIN] Full DI01 ODIN Package - xda-developers
Which will bring you completely back to stock as if it was out of the box, this process will take a couple of minutes including the download, extremely easy
Odin -
http://dl.dropbox.com/u/458252/Odin Files/Odin3 v1.3.exe
Stock 2.1-
[ODIN] Full DI01 ODIN Package - xda-developers
Steps:
1. Pull battery from device
2. Open odin on desktop
3. Hold volume down button and plug device into computer
-This will place the device into download mode
4. Verify device shows up in odin
5. Click "PDA" button in odin (Never phone Area)
6. Select 2.1 file i just provided from where you downloaded it
7. Click "Start"
8. Enjoy your fixed /system partition!
you will be unrooted and fully bloated DI01 after flash. Good for returning to stock, or fixing your breaks.