I had anorexia nervosa while dating someone, and we broke up mutually when school started (different schools) and a month later I went into the hospital. Years later, he has told me how shocked he was that I would always be exercising, always be doing something, and never eating. He expressed great concern for me, but could never do anything about it.
I wouldn't blame a guy for breaking up with a girl because she had an eating disorder. It drags the relationship down, and, sadly, there really isn't anything he can do to help - she must want the help on her own.