Exercise bulimia and / or anorexia is often times a control issue, similar to over-eaters who lack control. Exercise bulimics feel it is one thing they CAN control and take it to an extreme and don't realize how thin they are (body image dysmorphia). Perhaps she is feeling she has no control over something going on in her life (job, health of a loved one, etc.)
There can be serious health consequences too. Has her menstrual cycle changed or ceased? BAD sign. Also, bone loss can happen with too much exercise and it is not reversible.
There's more going on than what's on the surface.
It doesn't help either when outsiders positively acknowledge the weight loss , giving her even more incentive.