My wifes ex is threatening to foreclose.?

My wife and her ex have an investment home in Washington state. In there divorce decree he was ordered to pay the mortgage which he has been doing as well as collecting the rental income. They have agreed to sell the house and it has been on the market for just three months. He is now saying if it doesn't sell by June 1st he will stop paying and foreclose. The decree states she gets 90% of the equity and they have disputed the sales price as he wants to just dump it for 30K below its appraised value. The decree doesn't state the house must be sold. Does any one know what action she can take to force him to keep up the mortgage. He claims he can't afford it but we know he takes home 10K a month as an airforce pilot and lives in base housing for free. He only pays $1535 a month for child support on three kids which he is behind on as well. When there divorce was final last summer there only debt was the mortgage ($2100 per month) and his car payment ($480 per month).