The police likely does not care whether the persons are "tenants" or "squatters". As long as there are no immediate dangers, the police does not want to get involved in a civil dispute about rights to property without a court order directing the police to get involved.
The friends likely are saying that they are paying rent to the original tenant or that they are somehow tenants of the original tenant.
The landlord likely will need to follow the eviction procedures specified in the Residential Landlord-Tenant Act to remove all persons from the apartment. The RLTA is at http://apps.leg.wa.gov/rcw/default.aspx?Cite=59 .
The statutes provide for a relatively quick removal of persons from the premises. To balance the quick removal, the statutes require that specific procedures must be done. The landlord's failure to follow the procedure likely will result in a dismissal of the case. The landlord may end up paying attorney's fees for the other side.
To avoid making mistakes due to inexperience, the landlord likely should review his specific facts with his attorney.
I completely agree with Mr.Nguyen's answer. This is not a crime it is a civil matter and you need an attorney to handle it without incurring liability.