Coming from a 350d he won't notice the difference and moving to the D800 I never thought "Wow this screen is amazing" coming from a 5d. It's perfectly usable for image review and checking sharpness in crops.
The 5D has the same LCD screen as my old 30D and I certainly noticed a huge difference when I upgraded the extra half inch and 700'000 (approx) pixels makes it much better for reviewing pictures and quickly checking focus etc I certainly wouldn't go back. Like I said that alone would be enough to have me wanting a mkii if I was buying today.