landlord tenant laws in WA state, tenant's rights against landlord claiming that they did not receive notice to move out

Landlord tenant: I was a cosignor on a lease. The tenants gave notice to move out and upon termination of the lease I was splely sent the bill and the tenants were not. They did not receive a statement of fees within 14 days. As co signor I thought I was only responisble if the tenants failed to pay. I then was sent to collections five days after receiving the bill although the letter stated I had 10 days to respond. The reason they gave me was because they could. My question is don't they have a ;legal obligation to send the tenants a bill then if not paid they come after me as the cosignor? - Is this your question? Add additional information