Bipolar and borderline personality disorder...how can they co-exist?

Like JG said, Bipolar is a mood disorder which explains the major mood swings, basically bipolar is a fancy name for manic depression. Borderline is a personality disorder. There should of been some formal testing on your friend, not like mri's and blood test but a psychosocial, she must meet the criteria according the DSM-IV. The best way I can try and explain it is... Bipolar is mood swings, usually the person will be depressed, down, tired some time(few days) then hyper or overly happy- think extreme moods, but borderline is you never knowing what you'll get hit with, one second there laughing the next minute there yelling and cursing, could become violent. It's rather confusing, but if your friend is not 100% positive get a second opinion.