Hi,
Sorry if this is a stupid thing to say but she doesn't have access to guns does she? If she's threatening a specific form of suicide then I'd be doubly wary.
It sounds like she's very depressed indeed, in which case, remove all pressure from her to do anything except get well. Acknowledge that she's completely demotivated and don't put pressure on her to do anything other than take good care of herself as best she can. Treat her like an invalid for a while. That's not pandering - that's recognition of how incredibly physically and mentally debilitating deep depression can be.
I'd start by taking 100% pressure off her. Tell her you realise she's very low and you want her to take a week off. She may have had months off already but there's a massive difference between weeks in bed with pressure to get the f- up and achieve, and a week in bed devoted to recovering from serious illness.
Make sure she has clean sheets and PJs, plenty of healthy snacks and water, no sugar except fresh fruit and dark chocolate, no white highly processed carbs as they make your mood run riot. Then help her fill her phone with her favourite uplifting music (you may need to steer this a bit if she has downbeat taste.) Get her a box set of her favourite comedy - again, the least dark one she genuinely likes.
At night, encourage a shower or bath and a short spell outside if she can bear it, then encourage her week by week to build up to gentle exercise, journalling, more time outside, plenty of vitamins and healthy food. Imo (having suffered clinical depression for decades) focusing on the physical symptoms first is far more effective. It helps you recognise this is an illness, and stops you hating yourself and encourages you, and others to take good care of yourself.