Did he ding the pan bolt holes while it was off? You should use a ball pien hammer with the round part against the gasket side of the hole area and smack it with another hammer. This bends the pan surface out or down. Then when the bolts are tightened it compresses the ding toward the transmission and squeezes the area between the bolts before the bolts tighten down. Do you picture what I'm saying?
When the bolts were tightened on the old pan they compressed the gasket and caused a waviness to the surface. You reverse this by dinging the pan. You put the edge of the pan on a solid surface as you smack them down.