I ran 500 ft with solid 12 wire and ran a 1/2 hp submersible well pump with no problems at all. That was 220 though so I was only pulling about 6 amps each leg but still I would think 12 wire would be fine.
You should have used 10/3. Always use 10 gauge on 220 volt applications. Yes, 12 gauge is all he needs for his pump since it uses 120 volts. It probably won't draw much more amperage than a ceiling fan.
