Actually, I was recommending a detailed assessment not for diagnostic purposes, but for purposes of treatment planning. Treatment would differ greatly depending on the results of the assessment - I do not see how anyone could plan a treatment without having developed a clear conceptualization of what is going on for the patient. From the limited information in your post, there are a wide range of quite different possible issues going on here, each of which would point to a very different treatment approach.
In terms of anxiety, I would only treat it as an anxiety disorder if it turned out that anxiety was a prominent part of the clinical picture. I do not see any indications in your post that this is a panic disorder or generalized anxiety disorder (each of which would be treated differently) so unless the assessment found these types of symptoms, I would not treat it as such. From your post, the most likely anxiety disorder would seem to be PTSD. If my assessment revealed information consistent with PTSD, I would consider treatment options very specific to PTSD. Personally, I like Meichenbaum's cognitive approach to PTSD, but I know many people also like Shapiro's EMDR. The Summer issue of "Cognitive and Behavioral Practice" has an excellent article which outlines in detail the decision-making process for developing a treatment package for PTSD.
However, from the information in your post, this could just as likely be more typical identity concerns of adolescence and, as you say, "teen angst". This would be treated quite differently, more as a standard treatment of adolescents with lots of normalizing of feelings, and possibly a group with other adolescents to get validation of concerns. One book I like in this area is "Cognitive Therapy for Depressed Adolescents" by Wilkes, Belsher, Rush & Frank. There is a chapter on "Special Issues Related to Sexual Victimization" which would seem quite pertinent here.
If the issues turn out to be more related to depression, the treatment might be a more standard therapy for depression. Or it may turn out that this is a straight problem with developing trust, for which developing a safe relationship with the therapist could be therapeutic. Then the therapist could help the patient to learn strategies for how to determine whether a person is worthy of trust, how much trust, etc. and try this out gradually in the real world.
Of course, whatever treatment plan ends up fitting the assessment data, it would also need to be based on the patient's goals for therapy. If you are able to get more assessment information which gives you a clearer conceptualization of the issues, feel free to post again with the information and people may have specific suggestions for you then. Good luck!