Hunger deadens psychological pain. When stress, anxiety and other bad feelings seem to make life unbearable, it is tempting to take an easy way out. This "easy way" may help one to feel better for the moment, but of course it does not solve any problems.
Anything that causes the ex-anorectic psychological distress to an extent where she has a hard time persisting may drive her to starving herself again. It is almost like an ex-smoker, the risk for relapse is always there.
Treatment with Prozac/Fontex/Fluoxetine may help to prevent relapses; discuss this with a psychiatrist.