I told her he is trying to reach me through her but she said he never asks about me. I believe he is stalking me through social media, using fake profiles to add me.
If you are still curious, go to one of the diagnostic websites and see if he matches enough behaviors. It's not medically trustworthy, but it might put your curiosity to rest. From what you relate, I cannot tell if he has BPD, bipolar illness, NPD (that is my gut diagnosis though), antisocial personality, or simple alcoholism.
I am a little troubled by your final comments. These indicate that you are obsessing over the breakup. Whatever he was, your own mental health is now the only concern. You haven't mentioned whether you are in counseling. It might be a good idea. Also, if you are to stay NC with him, which is the only sure road to recovery, you should find another best friend, at least for now. Detach with love from the one who is with him until you are sure that they are no longer together. She will understand. Most women avoid dating their best friend's ex.