I emailed CASA about something similar in 05/06. Basically asked "can I fill a chieftain on private ops".
Answer was yes, but you can only cost share with 6 or less people, otherwise you have to pay for the whole flight. Remember most corporate jets run as private
If he's paying for the whole thing then I wouldn't think it'd be a huge problem. Mind you, I'm not CASA so it may be worth him checking for himself