The 14 day notification requirement is a great affirmative defense, but you need to read it carefully and then notice that the last sentence of RCW 59 18 280 says (basically) no matter what the landlord can sue the tenant for damage done to the property by the tenant during the tenancy. This is a huge loophole. Landlords have better legislators.
The owner is attempting to assert "joint and several" liability. The contract likely says that is the case. This is a defensible position. The landlord's problem is that you had no notice for 8 months that there was anything wrong.
He can send you to collections. You have to respond as to why you do not owe the debt and then the collections agency has to report accurately that the debt alleged is disputed.
The real danger is that the landlord will lawyer up and you *could be* responsible for the landlord's attorney fees. That could be a lot more than $13K, hard to tell.
Can a lawyer help you with this? Yes. If you have been served with a summons and complaint, that is a very good time to locate and retain an attorney. Hope this helps. Elizabeth Powell