I didn't get this part that you couldn't work on Non O. Actually u could work on non O if u have work permit. I am on Non O (marriage base). I got workpermit and I can work. My friend
He can get re-entry permit at airport also at the time of leaving country.. Counters are behind passport control..He just needs to tell IO that he needs re-entry permit. IO will direct him to the room behind him
Legaly u can't work without work permit. The some people u mentioned they are surely working illegal. If they get caught. They will be jailed / fined/ deported back.
Re-entry permit desk is behind immigration counter, u have to cross immigration first
Let IO know at immigration counter that u need re-entry, he will do paper work accordingly and send u to room behind him with passport and departure card