I can't really add much to what others have said. I would note, though, that one of the problems with the diagnostic criteria is that they often seem to include stigmatizing language which can make it hard to read, and even harder to consider. It really is true that the best way to contemplate this question thoughtfully is to find a good psychiatrist to talk to.
You can always Google it. Thats what i usually do.
best to seek a professional. unless your friend has a medical degree, they are not qualified to make that diagnoses.
If you have concerns then you should consult with a doctor.
The diagnostic criteria for each disorder can be found in the dsm-iv. Try typing in dsm-iv for borderline personality disorder (bpd) or bipolar or schizophrenia.