you cannot,,, if she does not choose to try there is nothing you can do
she knows but at this point is making excuses
if you are slim and never gain weight you are kind of without any choices
if you have any spare weight ,,, tell her you want to loose the extra weight can we choose a menu to help "me" loose weight..... My wife kept telling me "we needed to loose weight" so I did 30 LBS... she gained 20 more..... She tries but there is too much excuse making ..
one hit any doctor of value will tell you exercise will not help
eat less is the only answer and less carbs and sugar
no special diet just less carbs
BUT if you tell her she is getting too fat .... she will be upset and she most likely will not loose the weight