Lucy, one added thing would be good to remember about your friend whenr´ther she is formally BPD is that you need to be able to set boundries with her. That means, 1) no calling late at night threatening suicide or you will call the cops to come and take her to a hospital. 2) you are not accountable to her about how and with whom you spend your time, although you do like her. 3) Family takes precedence. No exceptions. These are just a few of the things that you might have to put your foot down about. not doing so is disasterous for you, and not great for her either.
If you can't be consistant about holding to your boundaries, then you need to back out entirely. Again, I am less concerned about the label than the behaviour. You don't deserve the manipulation and the threatening and the irresponsible behaviour, regardless if it has its roots in a mood disorder or a personality disorder. It sounds like you have been a good, caring friend. Remember to care for yourself and your husband too. HUGGGS