In your situation, the operation and the aircraft must operate in the airwork or charter category (see the MR), if you are to be paid to fly his aircraft. As for your freind putting other passengers on the aircraft, that is up to him, as long as he continues to pay for the complete operation. The other pax must not contribute to the cost at all (much like corp aviation).
As for bizjet operations, any operation of an aircraft with a MTOW in excess of 5700KG requires an AOC. The crew must hold commercial qualifications, with the Capt hold an ATPL.
You are not breaking any regs as long as the aircrafts MR has the right cat of operation.
Good luck with the career mate.
Blue skies, and may all you takeoffs equal your landings.
