Medical Malpractice and Medical Negligence are synonymous. Medical Neglect could be another way of saying Medical Negligence. If that is what the person meant, there is no difference. If the person using the term Medical neglect used it to distinguish the case from a Medical Malpractice case (unlikely), then to me Medical Neglect connotes a failure to act.
I think you might be overanalyzing someone's choice of words though.