Is it possible to talk to her about HOW you may be able to help (if she needs/wants it, ofc?)
Otherwise, treat her as you would anyone else.

Warnings: don't criticise her food choices, eating habits, appearance, etc. Avoid discussing weight loss, diets, calories, nutritional info or working out in front of her, it may be a trigger for her.

When my mother learned of my AN, she criticised my eating habits, watched me eat, started a diet herself and would say to me, "Gosh, I haven't eaten since breakfast" whilst cooking dinner. I wish she was half as conscientious as you are as to what to say/do.