My boyfriend is being blackmailed by his mother?

My 20 year old boyfriend has been physically, emotionally, and verbally abused by his mother his whole life. Last Thursday, he tried to move out and was staying with me. The next day, his mother said that they found her mother's expensive ring among my boyfriend's things that he left at her house. She said that she would press charges unless he moved back in with her. My boyfriend's mother, father, and brother all had access to the ring in question back in California where it went missing, and back here where it was found. All of them have touched it, so all of their fingerprints are on it. She wants him in her house because she uses him for his workman's compensation checks and for the tax credits she gets because he is a college student. He is too afraid to report what she's done because of the abuse he's suffered from her before, and because he doesn't think anyone would believe him. Is what she's doing blackmail? And also, does his family have any type of case against him proving that he did something?