You can offer support. But eventually that will wear you down. She won't leave until she is ready to leave.

He's holding her manipulatively hostage with the suicide threats. She still thinks she can save him and that's part of why she stays. She can't save him. He needs professional help.

If it were me, I'd be blunt and tell her like it is. I'd probably lose the friend but oh well. Years down the line the friend would probably think back and kick herself for not listening, not getting out sooner.