How do I help my bff

Guys in need help, my best friend has PTSD and severe manic depression. She means the world to me and I will always be here for her but I don't know how to help her.

Today she made a cry for help and wrote her suicide note and this isn't the first time. Every time it happens I feel like my heart has dropped out of my chest so I can only imagine how horrible she is feeling. She is such a great person who doesn't deserve this but sometimes, when something triggers her, she goes deeper and deeper into this dark place and I worry she will hurt herself again.

She lives by herself and she doesn't have family that she can turn to when things get tough. Would it be overstepping if I asked her to move in with me? I just want her close and I want her to know that I will always be here for her. How can I help her when she goes into this dark place?