Leave her... she's going to become MAHOOSIVE!!!!
On the other hand, stay with her. Tell her to stop fucking about and do some exercise:
Sit ups
Running
Cycling
Buy her this...
She's such a MILF
My GF uses it every other day, has smoothies and baked potatos for food and doesn't really eat any crap. She lost loads of weight with the DVD and even toned her stomach and legs up more