Has anyone of you had to work with a person suffering from Piriformis syndrome? Did you device new exercises to manage the pain

You don’t manage the pain, the client’s doctor does. Unless the health care practitioner indicates no contraindications to exercise, stay away from pain. If you proceed, then you are diagnosing and treating a disease/condition, which is clearly outside the scope of practice for a trainer. (Stretching can irritate piriformis syndrome, too.)