Hi, I'd like to help you with your question.
It sounds like your husband is verbally abusing his daughter. If he is concerned with his daughter's weight, yelling at her and berating her is not going to help. He needs to either take a different approach or stop all together.
It seems that he is taking his insecurity out on his child. Only people who do not feel good about themselves abuse others. If your husband had a good self esteem, he would not need to put his daughter down and embarrass her. He could talk with her, let her know he is concerned about her health, then provide assistance to help her. He would work with her, not against her.
You have the power to help your step daughter. Tell your husband to stop abusing his daughter. He is hurting her deeply, making the situation much worse and he is damaging her psychologically. As a result of the abuse, she will learn better ways to hide her feelings and her addiction to eating. She will also continue to eat since eating seems to be where she finds comfort. She may take a long time to recover from the pain he is inflicting, or she may never recover. Emotional/verbal abuse is some of the worst abuse you can inflict on someone.
It is very important that your husband and/or you get your step daughter to a therapist. She needs to talk about how she is feeling and work on the abuse she has suffered. To find a therapist, contact your step daughter's doctor. Or you can search on line at http://therapists.psychologytoday.com/rms/.
Your husband also needs help. He is an abuser. He needs to learn better ways to handle his own feelings and how to stop hurting others. If he does not stop his behavior, he is going to destroy his relationship with his daughter.
Also, learn what you can about emotional abuse. Help your step daughter learn as well. The more you both know, the easier it will be to cope. Here are some resources to help:
Healing the Scars of Emotional Abuse by Ann McMurray and Gregory L. Ph.D. Ph.D. Jantz
Why Does He Do That?: Inside the Minds of Angry and Controlling Men by Lundy
Healing Your Emotional Self: A Powerful Program to Help You Raise Your Self-Esteem, Quiet Your Inner Critic, and Overcome Your Shame by Beverly Engel
I hope this has helped you,