CargoOne,
Hopefully before this situation occurs, hopefully before the owner buys the aircraft, someone in the aviation management would have explained that if the owner wants to fly in Europe he needs XXXX aircraft, but if he wants to go to NZ from Europe then he might need XXXX aircraft AND an additional crew that could be positioned, and rested, at the appropriate tech stop en-route.
Then if the owner wants to spend 25 hours in the aircraft it might be accommodated safely.
Hopefully this would have been communicated by the aviation manager and not simply ignored by the aircraft salesmen.
I know it was the salesmen who won with the previous aircraft owner I worked for. Then you have problems!