I believe because he accepted the downgrade he is not entitled to any compensation. Under the EU-regulations, only involuntarily downgrades are compensated with the fare difference. He could be entitled compensation for delay to arrival but I believe that lies around the 5-hour mark for l/h.

What CO/OnePass will throw his way is a different tale which is worth pursuing. Better to be persistent but be aware that you may not meet much understanding as he opted for a re-route. Why did he not just cancel and rebook?