I am not aware of any known connection between head injuries and the later development of eating disorders, however, I must say that I would not consider myself an expert in these areas either. There can be many effects from head injuries (TBI) and these vary of course depending on where and how extensively the brain is injured. One common one is the emotional lability that you mention. On the plus side, your daughter was young when it happened and the brain has an amazing ability to heal, so I would hope that there would be few lasting effects at this point. However, this is obviously influenced by the type of treatments received after the injury.
Clinically I have seen a correlation with perfectionistic tendencies and eating disorders, so this may be the real culprit, although it’s not that simple. Eating disorders can relate to many factors but it concerns me that she is not willing to get help. You may really need to push hard for this and being in college she most likely has access to student counseling services right on campus which would make it convenient for her. If she resists, you might consider seeing someone yourself to help you cope with your concerns, but also to help know where to draw the lines between parenting her still and letting her find her own way. Good luck to you both.
All the best,
Dr. Holly Counts