Should a Therapist Advise Divorce?

As a general rule, it is considered unethical for mental health professionals to give advice at all. Our job is to help you learn to make those decisions yourself, not to tell you how to make them.
Speaking for myself, I never say never, but I cannot imagine a situation in which I would give such advice. I have helped clients find clarity in their relationships, which has led them to seek a divorce. I have also had clients come to me seeking clarity in a divorce and end up deciding to stay together. I see my job as helping my clients find the path of the greatest mental and emotional health, wherever that may lead them.
I have, on occasion, encouraged a client to seek solitude for a time. Oftentimes clients are in a situation that is verbally and emotionally volatile. I may advise them to take a solo vacation, stay with a friend or relative for a few days, etc. In situations with the potential (or actuality) for domestic violence, I have certainly encouraged clients to seek safety, which usually involves leaving the marital home. Divorce advice, however, is for attorneys to give. As I said, my job is to help the client find the healthiest path, whatever that may be.