"Your results are back: it's negative"......um, is that a bad thing?
I would play with the 0s as 8s. Count then as 8 for everything and then it would be exactly the same, and probably easier to remember.
If he's playing with people who have never played Arboretum, though, asking Them to think of 0 as an 8 would be more confusing. After all, your scoring routes go in increasing order and zero is not more than seven.
So I agree that making 0 an 8 could be beneficial to experienced Arboretum players, but noobies would be better off with zero being the lowest number, I think.