This person may cost you your job, your sanity, your peace and relaxation - and still it won’t be enough.
This is a futile toxic Co-dependent enmeshed dynamic where YOU are actually inadvertently enabling her MH decline. You need to see her as a compulsive toddler who is demanding to eat all the cookies - and the more you allow / enable this the iller she will become.
She is not your friend. There is nothing reciprocating, mutual or caring going on here. There is demands and domination.
You need to ‘manage’ her rather than see this as a friendship. You will do the best for her if you put in boundaries. She should not call you at work. You are not available for her to mentally download - signpost her to The Samaritans for that. Tell her you are available at x time for y minutes per week to talk about light stuff. As PP said - she needs a break from herself and this will be good for her.
If you don’t want to implode in your own life or explode at her YOU need to take some firm actions, boundaries, consequences, deadlines etc.
She sounds like she has a significant PD if she has fallen out with everyone around her and won’t seek help. This isn’t something you have any skill or responsibility to deal with and your current unboundaried approach could inadvertently be preventing her reaching her rock bottom where she seeks professional help.
Your own life, relationships, career is being eroded here for nothing - she isn’t getting better by you indulging her impulsive and compulsive demands. She needs to learn to self soothe and emotionally regulate herself.