Commission of a crime requires both the physical act and the requisite mental state.  For example, some crimes require a specific intent.  If one has a mental condition which prevents formation of the specific intent, the person can't be convicted of that particular crime.The requisite mental state varies depending on the particular crime and the definition of that crime.  Without knowing the specific charges against your father,  it is not possible to know the requisite mental element for those particular crimes in order to determine whether or not borderline personality disorder would prevent him from forming the requisite mental state.  If the definition of the crime(s) with which he has been charged require a mental state which could not be attained due to borderline personality disorder, this would provide him with a strong defense.