Are those who have a history of eating disorders excused from obligatory Roman Catholic fasts? Say that a person has a history of eating disorders, such as anorexia or bulimia. Are they excused from obligatory Roman Catholic fasts like those on Ash Wednesday and Good Friday? I've heard of exceptions for physically demanding jobs or current illness, but not this.

Given that regulations for fasting are adopted per nation, I'd like an answer for the United States, although other nations would be useful supplementary information.
From a Question and answer page from the United States conference of Catholic Bishops:

Q.  Are there exemptions other than for age from the requirement to fast on Ash Wednesday and Good Friday?
A.  Those that are excused from fast and abstinence outside the age limits include the physically or mentally ill including individuals suffering from chronic illnesses such as diabetes.  Also excluded are pregnant or nursing women.  In all cases, common sense should prevail, and ill persons should not further jeopardize their health by fasting.

I know of no one that would not consider fasting to be risky for someone with an eating disorder. Those who will jeopardize their mental or physical health by fasting are exempt.