What can I do to protect him from her actions to take him out of my home?

My nephew is 18 and still in high school. His mother is threatening to take legal action towards me because he lives here. She also used to live with me but left because she was a troublemaker. She is saying she will take him out of my home because he doesn’t talk to her. All I want is him to graduate and go to college. She wants him to go back with her and he doesn’t want to. We live in PA.