DID is a specialized area of study. I would suggest that your therapist work with a clinician who has expertise in DD. In addition, there are numerous texts and courses available. In the appendix of my 1995 text there is a position paper written by the EMDR Dissociative Disorders Taskforce. That can help by offering a number of resources.
In addition to the written material it is very important to have interactive supervision in this field. A number of clinicians with many years of expertise are available for telephone consultation. Their addresses and telephone numbers are available through the EMDR Institute. Those who have given numerous presentations and trainings on the subject and are available for consultation to your therapist include:
East Coast
Catherine Fine, Ph.D.
Marilyn Luber, Ph.D.
West Coast
Gerald Puk, Ph.D.
Curt Rouanzoin, Ph.D.
They may also be able to assist him/her in finding a supervisor/consultant in the same geographical location.