I'm not sure if the USGA or the local state rules have priority in this situation.
But here a buddy of mine was in the same situation. He was a pro at
http://www.thegolfspace.com/riveroaks .
When he decided to go back to amateur status he had to wait 2 years and play in no events as a pro for that two years.
I suspect your buddy will have to wait 2 years after playing in this next event.