I'm wondering if her son even said anything to her. Perhaps she's claiming he did so because she thinks this lends credence and support to her snotty attitude.
Ooh, I didn't think of that. That sounds very likely. She adds in all the "health" rubbish because she's aware that she looks bad for being concerned only about appearances, so why not do the same and say the son went to her for help, instead of admitting that perhaps she's just being spontaneously judgemental?
The fact that the girl has been with him for 3 years and has gained the weight during this time, rather than in the last month or something, did make it sound odd before that the son was suddenly and out of the blue going to his mother for "help". The notion that she invented this part causes it to make more sense. Still just speculation, but it this way seems more probable.