What is wrong with my friend? Is she bipolar?

I think what she is feeling is anxiety. A lot happens at home so she has trouble with life. You said friends always leave her so she is obviously scared you'll leave her. If you are concerned about it talk to her and tell her you would never leave her (as you seem quite close). Sometimes she could have mood swings but everyone does. Put yourself in her shoes, because there is always two sides of every story. She could be texting that right as her mother is verbally abusing her, so, to vent her anger, she decides to get angry at you. Then her mother quietens down and leaves the room, leaving her to text more, so she decides that because theres no more shouting she'll leave it alone. If she doesn't want you jeprodising your friendship, she won't either. Just give her time. Please be careful with her, she sounds very fragile emoitonally, so be gentle and don't leave her out.