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Sorry for his situation.
Yes, he has a cause of action for "unjust enrichment
"UNJUST ENRICHMENT - is a general equitable principle that no person should be allowed to profit at another's expense without making restitution for the reasonable value of any property, services, or other benefits that have been unfairly received and retained.
Although the unjust enrichment doctrine is sometimes referred to as a quasi-contractual remedy, unjust enrichment is not based on an express contract. Instead, litigants normally resort
to the remedy of unjust enrichment when they have no written or verbal contract to support their claim for relief.
Unjust enrichment is a benefit by chance, mistake, or another's misfortune for which the one enriched has not paid or worked and morally and ethically should not keep. If the money or property received rightly should have been delivered or belonged to another, then the party enriched must make restitution to the rightful owner.
So, I would file suit against the property owner for unjust enrichment - that the property owner is unjustly enriched at your nephew's expense and effort.
Since they don't have a contractual remedy - this is a quasi-contractual remedy.
It's somewhat like the "restitution".
Now what your nephew could recover is the cost to build the structure - not the land - just the structure.
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