You are being subpoenaed because they believe you have information or documentation helpful to them. You have to go and testify, but you are not required to violate the patient-doctor privilege, assuming that the nature of the counseling you did and the nature of your role allows for the application of that privilege in the first place. If the privilege applies, you can refuse to testify about any specific item covered by the privilege, but cannot globally refuse to testify and must testify about any fact not covered by the privilege. Even though the plaintiff should have been advised that you were subpoenaed, a good idea would be to advise him (or his attorney, if he has one) that you received the subpoean, so the plaintiff can be prepared to raise objections.