I completely understand this concern. Trying to change bad eating habits and lose weight can be very discouraging. It sounds like you have expressed your concern to your mother and
I think this is important but try not to fight with her about it. Try to stay on the supportive and encouraging side. You could try encouraging her to go on nightly or morning walks (as her physician allows). Dieting is hard to do on your own. Would she consider a program like Weight Watchers where dietitians are involved? There are also fitness centers like Curves and YMCA's which help women with weight loss through exercise. Senior centers often offer exercise classes; and it can help to be with peers. I would encourage programs like this. Perhaps you could offer to check them out with her. Also, your mother's doctor could possibly recommend her to some kind of nutrition clinic at her local hospital where a dietitian will work specifically with her.
I'm sure your mother would like to lose the weight as well but one thing is for sure, weight loss is difficult. I would just advise you to be as supportive as possible and try to encourage some of these recommendation.