Many people happen upon this site searching for advice about how to maintain a friendship with someone who has borderline personality disorder (BPD). It can be very hard to be a good friend to someone who experiences frequent emotional ups and downs, intense anger, and high risk behaviors like suicidality and self-harm.
That said, it can be done. It is possible to be a good friend to someone with BPD-- someone who is there for the long haul and doesn't run away when things get tough. But it requires work and an ability to balance your own needs with the needs of your friend.