This note is being written in defense of this book, which I and other fellow peers who work with adolescents in crisis found to be quite insightful and useful. I agree with the author that it is not appropriate to give a Borderline Personality Disorder (BPD) diagnosis to a teen. At the most, I would give "traits of BPD", but at this developmental stage it is truly impossible to say with certainty that a client is personality disordered.
As per DSM-IV-TR, Fourth Edition (APA), pg. 687 of Personality Disorders under Specific Culture, Age and Gender Features reads: "Personality Disorder categories may be applied to children or adolescents in those relatively unusual instances in which the individual's particular maladaptive personality traits appear to be pervasive, persistent, and unlikely to be limited to a particular developmental stage or an epiosde of an Axis I disorder. It should be recognized that the traits of a Personality Disorder that appear in childhood will often not persist unchanged into adult life. To diagnose a Personality Disorder in an individual under 18 years, the features must have been present for at least 1 year."