Since she's brought up her son's weight, I might ask questions about her son's problem - but not mention hers unless she brings it up.
My history on the subject:
I'm 4'9", and 160ish pounds (size 14-16). Mom is 4'11", 165, size 16-18. Dad is 5'8", 155, size 34 pants at age 56, with thyroid issues (he's the only one of us with those issues, AFAIK). Mom is on me constantly about weight loss programs and exercise. We live across the country from each other, and Mom will nag me over the phone. Dad used to nag us both about it constantly. Both drive me up the wall.
I know they're concerned because I'm getting heavy at a young age (26). Part of the reason I'm so heavy is from medications I took in the past - even with trying to eat right and going to the gym (with personal trainer guidance) or doing cardio at home *EVERY* *SINGLE* *NIGHT* for a *YEAR*, I didn't lose a single pound. That was very discouraging, and makes me not even want to try sometimes. So when Dad, who's in great shape, even with the thyroid problem, starts in, I want to scream at him, "I've tried!!!!" When Mom, who's heavy in part because of medication, and in part because she eats whole cakes at a sitting (I wish I were exaggerating) and rarely exercises, starts in, I just want to laugh meanly and say "you first" (though I don't).
DBF is 6' and 280. I said absolutely nothing about his weight until he brought up recently that he hates that his size 44 pants are getting tight and that shames him. We both want to change and are ready to make some changes - we both bought different salads we like at the grocery store, stocked up on lean meats and easily cooked meals since we eat fast food too much, and we're checking out gyms on Friday. We'd check them out earlier, but I have a foot injury from over a week ago that I'm just now able to walk on.
Bottom line - you might be able to show concern for her kid, since she brought it up. But trust me, she probably knows why she's heavy, and she probably knows exactly what she needs to do to lose weight. Everyone in her life, except you, has very likely given her all kinds of hints, some good, some bad, but mostly well-intentioned, and they haven't had a positive effect. The idea of trying to lose even a moderate amount of weight can be overwhelming, and having friends and family harp on it is infuriating and embarrassing. Eventually, she may decide to do it on her own - but she needs to make the decision herself, and she'll probably ask for support from her nearest and dearest when she does. My advice is to wait for it.