I agree with the OP. A relationship is a two person arrangement. In fact, I think of relationships as contracts. Each person has a responsibility to the other. If as her mother says, Hallie offended her, she has a communicate her feelings to Hallie, rather than just ending the relationship, or cutting her off. Same goes for her friends, if somehow they were offended by her words or behavior, it's their responsibility to speak up.
Hallie isn't a mind reader and doesn't deserve the lack of commitment that her mother and her friends showed her. No one can make changes in their behavior if the people in their lives don't communicate with them. In my opinion, if her friends were good and true friends, they would have worked to repair the relationship, and communicated their feelings rather than just bailing out on their friend.
Hallie, I commend you for your courageous decision to make a drastic change in your life, that is sure to improve your health and the quality of your life, going forward. My recommendation to you regarding your relationships, though, is this: when you feel your friends pulling away, ask them why, try talking to them. But if they are unwilling to give you reasons for their behavior, don't keep pursuing those relationships.
I can tell you from experience, that you can't fix what you don't know is broken. If they are unable or unwilling to articulate what it is they want from you, or what you've done to upset or offend them, they cripple your ability to improve the relationship. It's my experience that someone who behaves this way isn't invested enough in the relationship, and it's my opinion, that you're better off without them.