Ahhh i completely understand where you are coming from. I have a friend in a very similar situation. She too has suffered from Anorexia in the past and i know not being able to excercise really freaks her out. I often try and eat with her, always healthy stuff i must add (which is not really me!). The only advice i can offer to you is to just be there for her, reassure her and try and occupy her anxieties, that way it keeps her mind off wanting to exercise. I have learnt so much about this illness and i now know any signs my friend shows as sinking back in to her old habits, try your best to pick up on these and never let it get a hold of her again. She needs to understand that to lead a normal life she must be a healthy weight and if you can help her believe that things will be ok! Sounds like she is very lucky to have you! Be patient.