Is a false accusation of borderline personality disorder grounds for defamation of charachter

Is false accusation of borderline personality disorder grounds for defamation?

If you're asking about grounds for a lawsuit, making untrue statements about another which damages his/her reputation is defamation of character (Libel if written and slander if spoken).

There are a lot of unknowns in answering this question. Assuming they cannot prove the statement is true (best defense), that the person it was said about is not a public figure, that the statement was made in a way that a lot of people are aware of it, That it damaged the person some way and the court rules that having a personality disorder is somehow damages your reputation, there may be grounds.

The real answer though is to get an appointment with a lawyer, let them review all of the facts and they will give you an assessment of your chances of winning the case.