Gotcha... I dont mean to come across as a cheuvanistic ahole, frankly I dont care that she is overweight (not obese by any means), just tired of spending hundreds on drugs and docs and hearing the complaints all day that it isnt coming off when the solution is so simple.
You should probably make the point to her that exercise will increase her ability to be independent and will delay the time when she gets sent to a nursing home.
It also allows her to be more steady on her feet and reduces the risk of falls, which is the biggest reason why old people end up in nursing homes.