Qigong is not safe for mentally or emotionally unstable people unless they are under the care of a licensed Traditional Chinese Medical Doctor because the qigong will be balanced with diet, mundane practices, herbs, and other specifics the doctor sees fit depending on the needs of the patient.
From experience, qigong is a great stress reliever on the physical, emotional, and mental level, and helps me with feeling vital and productive, but I have had many years of training under instructors and when I had emotional problems I saw a therapist pro bono and had long emotional talks with my closest friends which helped me heal.
With all of that said, qigong for general health is an excellent idea! But for mental health, she should steer clear unless she is under the supervision of a qualified DMQ (Doctor of Medical Qigong) and trusted and loved therapist (I personally don't agree with a lot of the therapy I was raised up with, maybe it's improved, but it helps certain people and maybe your cousin is one of them it can help). I would also say diet is a great step in the right direction. A licensed and compassionate Naturopath and/or Nutritionist would be good for this.