No, he should observe Spanish law. You’re right, and I appreciate your clarification. But couldn’t the matter be litigated in some other venue?
Long delay on answer. Sorry.
I am guessing most other venues will be no better. The concept of “holder in die course” is a common law right and should not be diminished even for Nazi looted Jew owned art. That sounds terrible. But the alternative is worse. Did our ancestors steal the land we now occupy? What about the diamond you wear? How many transfers should stolen goods be traced and retrieved from? How about corrupt judges and lawmakers who decide something was stolen way after any witnesses have died?
Remember if there was knowledge of the theft or reason to be suspicious then “holder in due course” does not apply.