In general, therapists should not diagnose someone who is not their patient. Therapists are bound by ethical and legal guidelines that require them to maintain confidentiality and privacy for their clients. Diagnosing someone without their consent or without an established therapeutic relationship would violate these principles.
Therapists are trained professionals who use their expertise to assess and diagnose their own clients or patients based on the information provided during the therapy sessions. They gather relevant information, conduct assessments, and use their clinical judgment to arrive at a diagnosis when appropriate.
If a therapist believes that someone they are not currently treating may have a mental health issue, they should encourage that individual to seek professional help from a qualified mental health professional. They may offer general information about mental health resources or provide guidance on how to find the appropriate care, but they should not provide a formal diagnosis for someone outside of their client base.