How to I get my fathers stuff out of his house, when his soon to be ex wife calls the cops if I “trespass”?

The house is in Washington State, my father is working over in Florida for months at a time and cant leave for fear of losing his job. My father and his “wife” have been given a 20 day eviction notice. If I try to go to the property she calls the cops. Is there some sort of document or power of attorney form he could fill out giving me rights to the property? I simply want to go in the house and remove his personal belongings.
more info- The house belongs to my grandfather who is now living in an assisted living home. All of the bills are still in my grandfathers name. My father moved in as caretaker of the house with no lease. While he was gone working in florida his wife moved in and registered her cars there, and gets her paycheck sent there. With washington law, that give her rights as a resident, so we have to go through the eviction process to get her out, and since they are still married we have to evict both of them. If he was in town he could simply go in and and get his stuff, but with his work that cant happen. She has already starting selling his tools and he has already lost a few thousand dollars in belongings.