I would turn your phone off while you are at work, only check it on your break or lunch, once or twice a day and delete much of what she texts as if it were junk emails. If she is calling your place of business phone, that is unacceptable and she has to be told she cannot call that phone, it is for the business only.

I would decide when you want to see her and how you want to support her. If you are taking her out, fine; set a once-a-week date/time and the two of you go out and "catch up" for the week. Maybe make a little list of counselors or grief support groups she might be interested in joining in your area. You are only one person, not a professional (I don't think?) and it is not your job to take the whole of her messes on, just a one-person, friend's portion. She has to do her own grief work, you cannot do it for her but it sounds like she is doing the angry portion with her boyfriend and that's not somewhere you want to be.