He could always start working in Photoshop non-linear history mode. To get this:
1: Open PhotoShop
2: Find your History palette
3: Click the 'options' button in the palette (A picture of a down arrow and 3 horizontal lines)
4: Click History Options
5: Click on 'Allow Non-Linear History'
6: Check out the other options. I like to have the top three on (Auto First Snapshot, Auto New Snapshot on Save)
7: Click Okay
8: Start using!
One note; I do find that while in non-linear mode, sometimes Control-Z/Control-Y don't work for some reason. You can still use the menu or History palette when this happens though.
Takes a while to get your head around, but it can save your ass from hour(s) of redoing something because you thought you were smart and did a destructive change as opposed to non-destructive.
I would recommend running Smart Objects more often as well. While Photoshop still doesn't have this 100% flawless (Read: canvas size snaps right against the smarted-object and a bit awkward sometimes to work with), it again gives you another level of non-destructive control.
Also, if you're applying filters to items (and running in CS3), run them on Smart Objects for more non-destructive fun.
Finally, instead of doing adjustments on layers, create a new adjustment layer directly above the layer you want to apply it to, set it up, then hold down Alt and click the little line between the two layers (Links the adjustment to the layer below.) If you need to do an adjustment to a set of layers, group them into a Smart Layer.
Finally; Never crop or delete parts of an image; Always mask it!
Hope these tips help.