I've been in this exact situation, OP and it's exhausting. I'm sorry to say I let it go on for far too long, so I'm going to give you advice based on what I wish I'd done!
I would send a kind but clear message/email along the lines of
"Dear Friend,
I am sorry that you're going through a difficult time and I want to help, but I am starting to feel out of my depth. As your friend, I am asking you to please re-consider getting some proper professional help.
I've done a quick search and the closest Mental Health Drop in to you is (details here)
You can self-refer to talking therapies here (if her area has this) OR
you could talk to your GP and ask to be referred for talking therapies and/or consider trying medication
I can't always answer the phone or respond to messages when I'm working or away, so I also wanted to mention these 24 hour helplines you can call or text (details here)
I care about you very much, and I am happy to listen and support when I can, but I can't be on call 24/7 and even if I could, I'm not an expert.
I hope you understand that I have your best interests at heart."
If she reacts badly initially, that's not your fault and it also doesn't mean she won't reflect on what you've said later when she's had a chance to calm down.
Once you've sent it, you need to follow through. This means not picking up the phone when you're working, if you're out or away. If you're out and she tries to contact you you could send a quick text saying you'll call her when you can. When you do speak to her, continue to encourage her to seek professional help and if she dismisses it say something along the lines of "that's your decision, but if you won't get help I'm not sure what I can do to help you". If she threatens suicide, you absolutely must alert the emergency services. She may not like it, but she needs to understand that you are not a crisis service and she's leaving you no other choice. Do it every time, it's important to be consistent.
It shouldn't be a straight up choice between ditching your friend completely or being constantly available to her. BUT if she won't accept reasonable boundaries then you may have no choice.