What is the statute of limitations for paying off a car in WA?

My boyfriend is currently making payments on a vehicle that he does not have. When he was married, he and his ex wife bought a vehicle and when they got divorced she was awarded the vehicle and was supposed to pay the bill...and she didn't. The vehicle was voluntarily repossessed and sold at auction. There was still an outstanding balance on the vehicle after being sold at auction.

My boyfriend was employed, but his ex wife was not. The finance company sued for the balance of the vehicle, and because he was the only one with an income the court awarded in favor of the finance company and made him pay. In the divorce decree it states that his ex wife was responsible for the vehicle.

It has been several years since the divorce and the court awarding the finance company payment. He would like to sue his ex wife for the money she should have been paying for the vehicle. He is currently making payments of $75 per month for the court's ruling.

What is the statute of limitations in washington state for something like this? Does the time for the statute of limitations start from the date the divorce was complete or from that day the finance company was awarded payment?