Why does he think civilians shouldn't have semiautos? Perhaps we could address his specific arguments.
If it's about rate of fire, point out that revolvers are every bit as fast, and that pump actions and lever actions are fast enough so that it doesn't make a significant difference. Heck, I think I can fire my Ithaca 37 quicker than I could a semiauto
.
If it's about capacity, perhaps you could could compare the pellet count of shotgun shells in a pump action shotgun to the magazine capacity of submachineguns, and then there are also the edge cases, such as Remington's pump action that takes AR-15 magazines (including drum magazines?), and pump action AK's with drums.