Good question but pretty complicated.
Generally a psychologist has an ethical obligation to attempt to legally resist a subpoena unless the client has requested the psychologist testify on his/her behalf. And even then, it's important the client understand it's an "all or nothing" situation. For example, a client might want a psychologist to testify at a custody hearing about sessions that have focused on parenting skill development. But if asked, the testifying psychologist would have to answer questions about the client's substance abuse struggles. So IF BL did have a psychologist, and he did ask him/her to testify, it would be all or nothing in the same way.
Note I said "legally resist" above. The first step for a psychologist receiving a subpoena is to talk to an attorney. State laws vary and laws change. So sound current advice is needed. (Most trials will be state level.)
Rarely (IME) would a prosecutor attempt to get a psychologist to testify a client confessed to a crime. That's just not the way things go and probably wouldn't be allowed because it's hearsay. But if court-ordered to provide records or to testify, then most psychologists would comply. Some wouldn't and would face a contempt charge.
The following link is for training but has more info for those who are interested. I'm afraid to say more for fear of being off topic!
Protecting patient privacy when the court calls
JMO