Ms. Border's post is correct. As further clarification, your friend doesn't need to leave a copy of the Proof of Service with "them" (respondent's attorney), and "they" (your friend) will have to sign the Proof of Service.
The law in Washington State is that anyone over the age of 18 who is not a party to an action may serve the opposing side with documents. A Certificate of Service/Return of Service must be signed and dated and indicate where it was served by the person who is the server. This must be filed with the court.