Neighbor just called and asked about plugs for a basement he is finishing. He ran 20 amp breakers and 12 ga wire to his outlet boxes. He is questioning weather he can use 15 amp or does he have to use 20 amp outlets. I believe that 15 amp outlets are fine as long as he is not plugging in "20amp" loads (these plugs won't fit into the 15amp outlets)

Having a brain twist here want to ans correctly.

Thanks

Sounds like he can use either. The standard 15 amp outlet will only accept a 15 amp fixture such as lamps, TV, DVD and the like. If he installed a 20 amp outlet it will accept the 15 amp male plug but also accept a 20 amp and he has the wire and breaker for that.
Wire sizes, gauges, for breakers (http://www.greatinspector.com/faq-elec-wiresize.html)

Ballengerb1 is correct, he can use either, if he has some 20 amp equpment ( they have a 20 amp plug that will not plug into a 15 amp plug) But if he does not have any such equpment, than a 15 amp plug will do fine.