What he should have done is seperate them as he planted them. But too late for that, so yes, thin them. Although I'm not sure you'll get good polination with only 12.
Stacey

I would thin them. You should have one plant every 8" or so. If he is only planting the twelve plants, I would put them in 2 hills of 6 plants each. You'll get better pollination than if you leave them in one long line.

Thanks guys
He has them in two lines, 6 or so inches apart.
He left half his box open, which he will be planting soon. He wanted to spread his growing season.
I'll tell him to snip the buggers.

Sweet corn needs to direct seeded in the ground. If I were betting, I say he won't harvest anything.

Then why would it be sold as seedlings?
Also, why would you bet that he wont harvest anything?

When he thins them, just cut the unwanted plants with scissors. If you try to pull them out, you may get both plants or at the very least, do damage to the roots of the plant that you want to keep.

I'm no expert (this is my first year gardening) but I am also growing sweet corn, and my research indicated that it would do better planted in a block of at least 16 plants (4 rows of 4 plants). I was under the impression that it's best to direct seed it, as well, but... although I'm just a newbie... if the transplanted plants are growing well, I don't know why they would not produce corn. Unless it was a problem with pollenating due to how the plants were arranged.

I plant mine at two seeds per peat pot and plant out when likely to be no danger of frost- end of May. Always leave two per planting position if they are good plants but space at 15 inches instead of 12 inches for singles but always feed very generously. Usually put in lots of manure in Autumn, put 100gm/sq m of potato fertilizer ( 15N-15P-20K ) and also 100gm/sq M of Fish, blood and bone. Never fail to get a good crop even in North of England and in a good year get four cobs per planting position. They are very hungry plants!

UPDATE
Plants are 7 feet tall and popping out hairy corn husks out of a bunch of joints.
I think he'll get some nice yummy corn for the grill

I live in central Wisconsin. It is often very difficult to get the garden planted before Memorial Day, and my corn season is often cut short. I think I am going to try starting corn seeds in "cow pots" next year and see if that happens. I can bury the pots (made from cow manure)right into the garden and not upset the roots of the plants at all.

This has been an incredibly wet spring, and my garden is still not in. We have heavy clay soil that I am constantly amending. When it gets too wet it cakes to the tiller, so I have to wait for it to dry before I can till and plant. I'm not even sure it's worth starting corn from seed at this point (but I'll probably try anyway).