Do you visit there a lot? If you do, I would say to try and cook dinner a few times a week if you can. If she works, you can pack her a lunch. Phrase it as a "you help me out so much, I want to help you" or "I am starting to love cooking!" Or you can ask her to be your "exercise buddy" or something of the sort, and frame it as you needing motivation and help to stay on track. I would say don't do it as it's already happening or too late to help (like when she's already cooking).
Of course, as everyone else has says, it's not up to you to make your mother do this, and I'm nowhere near as experienced with dieting and staying on track as most people here probably are.

But it sounds like she maybe just needs help, and like she would want to work on things with you, so if you can keep each other motivated, maybe that would work?