If you type into Google “is working with past lives ethical”. You will find an article by Medical Doctor Gabriel Andrade published in the National Library of Medicine who says past life therapy is not ethical for these reasons;

First, it is not evidence-based. Past life regression is based on the reincarnation hypothesis, but this hypothesis is not supported by evidence, and in fact, it faces some insurmountable conceptual problems. If patients are not fully informed about these problems, they cannot provide an informed consent, and hence, the principle of autonomy is violated. Second, past life regression therapy has the great risk of implanting false memories in patients, and thus, causing significant harm. This is a violation of the principle of non-malfeasance, which is surely the most important principle in medical ethics.

I entered into correspondence and you may be interested in what I said.
The professional use of past life therapy is called regression therapy, and a single worldwide standard of therapists exists controlled by the Earth Association of Regression Therapy. The association maintains standard of regression therapy, including the type of questioning used, in the same way that medical council maintains the standard of doctors. You will find a summary of research with past life therapy on my website https://www.regressionacademy.com/past-life-research.htm.

It may also be worth reading a book by medical doctor Peter Mack who use past life regression and got amazing results with one of his patients https://fromtheheartpress.com/healing-hurt-within-regression-therapy-boo...

The use of hypnosis including current life regression was accepted in 1950s in the UK and USA. I have a chapter on using past life regression therapy in the book Clinical Hypnosis Textbook by Professor Ursula James who uses it to train medical doctors in Oxford and Cambridge. 

It may be worth reading The Trances People Live by Stephen Wolinsky. He suggests people live most of their lives in trance - watching tv, reading a look, thinking of the future, thinking of the past. It’s only when they are in a mindful state of the here and now that they are out of trance. So talking to friends who are not trained to ask the right questions is far more dangerous that being in therapy with a professionally trained therapist working with the right intent and asking open questions like “tell me more” and “what happens next”.

When a therapist is exploring a past life and repeats back what a client has said but makes a mistake the client correct them which suggests the client is in complete control of the story even in trance. My book Healing the Eternal Soul goes into this in more detail https://www.regressionacademy.com/past-life-regression-books.htm

Do past lives need to be proved to be real before using them? Jung used dreams for many years in his therapy but did not need to prove they were real before using them. just like past-life regression therapy, are not very effective. However, regression therapists are very careful and explain to clients that if a story emerges in therapy that sounds like a past life it may or may not be real but it comes from them, and like a metaphor is very helpful in the healing process.

Although the research is not complete and published a medical team in Italy have found that using MRI scanners during a past life recall a completely different part of the brain is activated than that used from imagination and current life memories. So perhaps in a year or so when the research has a larger sample and published we may all be thinking about past lives in a different way.
https://www.regressionacademy.com/articles/past-life-memory-research.htm

Hope this is of help to you, Andy Tomlinson

Author's Bio: 

Andy Tomlinson is a psychology graduate and registered psychotherapist. He has also been trained in Ericksonian hypnotherapy and regression therapy, and is a qualified life between lives therapist from the Michael Newton Institute. Andy has run an internationally renowned private practice dedicated to regression therapy since 1996. He is the Director of Training for the Past Life Regression Academy https://www.regressionacademy.com/regression-therapy-training.htm and a founder member of the Spiritual Regression Therapy Association, the Earth Association of Regression Therapy and the Society of Medical Advance and Research with Regression Therapy