My grandpa and his wife recently got a divorce?

They own a house that was agreed to be sold and split 50/50 they also came to an agreement that she could live in it till it sells as long as she is the only one to live there we have found out she has her boyfriend and 5 grand kids living there. How does my grandpa go about getting her out of the house as she did not hold up on her part of the agreement and the realtor also feels the house will not sell with her and all the other people living there. He lives in Colorado The house is in Teller county and he lives in El Paso County.