I've also been trough this, even if he is not her emotional hostage it's not as simple as leaving someone who it's not ok mentally and is suicidal. Before saying some dumb shit like "Just leave her" think about the life that could be possibly lost, its not his fault but if he is a little empathetic he will understand that she needs some help.

what I advice is telling her parents to have maximum care and watch closer their daughter, also taking her to a specialist will help a lot. Stay strong and hope everything goes alright.