Hi, I'd like to help you with your question.
It sounds like your friend's parents have some issues they never worked out. They are harming their son by treating him this way, but he can gain the power to move out from under their control, with help. It seems he does have some handle on how dysfunctional this is just in how he feels about his mother.
There are many resources to help. Some of these are going to address co dependency in drug addiction and other circumstances, but the same principles apply. Here are some recommendations that will get you started:
Let Go Now: Embracing Detachment by Karen Casey
Codependent No More: How to Stop Controlling Others and Start Caring for Yourself by Melody Beattie- this book may give you and your friend some insight into his parents behavior. It will help him understand the situation better so he can find ways to break free.
Facing Codependence: What It Is, Where It Comes from, How It Sabotages Our Lives by Pia Mellody, Andrea Wells Miller and J. ***** *****
Love Is a Choice: The Definitive Book on Letting Go of Unhealthy Relationships by Dr. Robert Hemfelt, Dr. Frank Minirth and Paul Meier M.D
Codependence / The Dance of Wounded Souls by Robert Burney
Let me know if I can help in any other way,