quantum has some good ideas so you don't start 'lecturing.' asking her questions every now and then (taking one or two at a time during a conversation - when she brings up the weight issue).
my mom is actually overweight, but i never bring up the topic. she usually does when a dessert is offerred or something happens that she starts to feel out of control. unfortunately, over the years, my dad has sometimes joked with her about it. this really has hurt her feelings, so she is extra sensitive to joking.
she tried weight watchers, jenny craig, all kinds of natural and unnatural diets, etc. etc. and found that her 'natural' body weight tended to shift back even after exercise and diet. now, she has found gardening to be sort of a relaxing and meditating kind of thing. and, even if she is not completely satisfied with her body, people often compliment her on her garden and things she can do. she isn't ugly, by any means, but she never seems to be satisfied with her looks. (i can't understand this because often women who are slightly overweight are more 'voluptuous' reubenesque figures).
maybe hollywood is the culprite and also, fashion designers (although many are starting to design for larger sized women). what is kinda strange to me, is that i am much taller than my mom and naturally much thinner (taking my dad's genes or something). i haven't struggled with my weight that much, but at times when i was younger wished that i had the fuller bust. which ever side you are on, you always want something on the other. i guess as women get older they naturally gain weight everywhere - it kinda equals out the playing field.
when i was in my 20's, i could still eat anything and not gain. in my thirties, i noticed weight gain around pregnancy was hard to get rid of so i started bicycling a lot. now, in my fourties, i can't eat bread. at all. one banana split and i gain 10 lbs instantly. so i rarely eat ice-cream and try to take the burger out of the bun. you can ask for diet plates, but the trick is to eat slowly whenever you eat (making yourself take a certain number of chews). you see, i know this because instead of overeating, i was one of those skinny diet concious girls when younger. if you copy what they do and eat little bites. saving a portion of your plate for the birds. things like that. you'll probably at least maintain the weight you are at.
exercise is the number one thing to make a person feel good about themselves, imo. even if you don't lose weight per se, you are toning your skin. so, you don't, in effect, look fat. it doesn't hang in folds. buying a weight machine, trampoline, etc. couldn't hurt. then, when complaints are started - you just point at the machine and tell her to do a few and you'll treat her to some soup and crackers. (this is sounding sad) but really, to help someone, you kind of have to eat the same diet (and hide your cookies). my dad has learned to not crack jokes, to eat similarly, and to encourage my mom by taking her out more and making her feel personally that he likes to be with her.