My stepson's mom called my husband saying she stabbed her boyfriend. What should we do?

She called him this morning crying that she stabbed her husband and that she was scared that she was going to go to jail. She is the custodial parent of the child. What should we do? We do not want to send him back to her because we are worried about his safety. We want to get custody of him. Should we call CPS or the police or what?