Hello, the BPD of your friend actually who diagnosed it. Please consult with a psychologist and they may refer to a psychiatrist later if required. There could have been some situations in his life could have been the reason for this particular behaviour. Please consult an expertise in your city and help your friend to come out of this at the earliest to life peacefully and enthusiastically.
Borderline personality disorder is primarily treated by psychotherapy. the treatment of choice is dialectical behavior therapy (DBT). Medicines have some role and may give upto 20-30% improvement. Unfortunately, there are very few people who can provide dialectical behavior therapy. If you were to be in Delhi, I could have given you the therapy but that is not the case. It will take atleast 8 sessions, once weekly, each session lasting one hour. If you are planning to move to delhi sometime you can contact me at stayhappy.prakash@gmail