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The best thing you can do for your friend is to be the best friend you can. Friendships are irreplaceable especially in times of extreme emotions. Help her decide if this relationship and other stressors are becoming more than what she can handle. You can help facilitate or encourage her to find a therapist that can help sort this out. Sometimes when people are depressed this can come with a total lack of motivation. First listen to her and offer impartial support. Then suggest that she find a therapist who can help her sort out her life and look at her choices. She may be the caregiver type - cares for others first and not for herself. Sometimes that reburies someone close encouraging them to address their needs because they don't always do so.
I would do a little research for her. Find a therapist in your area and this will help to advocate for counseling. You can even go with her.
Let her make her decisions. It is up to you if you choose to support them. Most importantly listen