Oregon has laws that favor squatters and deadbeat renters. He'd better get a good lawyer who knows how the laws work. I do know that a squatter has to live there 10 years before he can claim ownership under adverse possession. However, getting rid of them can be tedious and expensive.
Alaska is similar.
A couple squatted in an abandoned shack down the road from us. Cost the owner(lower48) $20k to buy them out.
