"Crooked teeth" will not kill you, being overweight might.
Obesity and Overweight: Health Consequences | DNPAO | CDC
Wanting your partner to lose weight is not only about looks, it is about their health. This is especially true if you are thinking in the long term.
To me, this is more like asking a SO to quit smoking and buying them an electronic cigarette.
My suggestion: start training for a triathlon; you'll be training and getting in better shape, I'm sure the enthusiasm will rub off. Then you can ask her to help *you* eat better and keep on *your* exercise routine.