I'd caution that suspecting (out loud) that she might develop an exercise disorder would be one of those insulting or belittling things you were worried about (either because it seems like a cheap shot based on the anorexia diagnosis, or because this might be one approach to actually getting out from under the anorexia by exerting control over her body).

Likely a better approach to this concern would be to silently watch for those behaviours developing and worry about it if and when it actually does happen. (Note that refusing to help her with training and diet means she gets this help from someone who is not watching out for the possibility of exercise addiction).

There are a few approaches that might work for different people:

• Talk as though she doesn't have anorexia. Since you are aware, you can tailor your message to avoid saying anything seriously upsetting (i.e you can present the diet assuming control of diet is easy, or assuming control of diet is hard). I don't recommend this approach.
• Confront the issue directly ("Exercise is what tells your body to grow muscle, but food is what muscles are actually built out of, so without a caloric surplus your progress will be slow. I'm aware that this is probably a much harder challenge for you than most people..."). I don't recommend this approach.
• Ask her how she feels about discussing diet. ("Do you feel comfortable discussing diet with me? Feel free to say no. Also, don't feel constrained by your answer to this question; if later you start wishing you'd said no, just say that, and I'll stop."). I recommend this approach.

In any case, make it clear from the outset you want to be respectful about it.