I'm just a painter, what do I know, but it seems to me that since the jig is designed to build a certain amount of washout into the wing using a certain dihedral, wouldn't changing the dihedral by raising one side change the washout you got from that jig? Otherwise why jig both wings at one time so that they were an equal distance from the table top?

If his table isn't long enough, wouldn't it be better to measure the table top from the floor and let the other wing over hang that height plus the 2.5". Otherwords, if the table top is 36" above the floor, let one wing hang over the table 38.5" above the floor. Weight it down (pin it, clamp it, what ever) in the center so it didn't move and go for it. At least that way he could do one side at a time while the dihedral was correct. Then switch the wing and do the other side. I don't know, I think I'd probably find a floor and clear the dog off of it and use that instead. Then you could be sure. Just my thoughts.