This should be easy. I did this a couple of months ago to pick up a trip that needed to be covered. Since the FO is required, and you are taking passengers, he needs to get 3 TO's and landings in, and have the proper certificates, ie. ME Land, Instrument rating. That should make him legal to fly in the FAAs eyes. Insurance is something else. The manager of the aircraft should be able to talk to them to see if the sit in pilot has the quals they are looking for.