I am not sure there is a definite answer to this, it might depend on the attitude of indivdual customs and police officers you encounter along they way. It would help if you can minimise the number of border crossings done by the pillion without the owner being present.
You don't mention your route but if there is any shipping or flying of the bike involved it would be best if the owner was present as these clearances are always a lot more difficult than land border crossings.
A covering letter from the owner giving permission to use the bike, perhaps with a photocopy of his passport photo page on it might help.
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