He is obviously not a real man then… is he Brad Pitt himself that he is ready to kick her to the kerb over something that is inevitable for most women??? 
Stretch marks are a sign of child bearing hips… she should feel proud of her body.
BTW i don’t have them… even after having 2 girls… and i’m pretty sure it’s related to my intake fo flax seeds/ flax seed oil … 
Nothing can get rid of stretch marks, but there are many lotions and potions on the market to make them less obvious and to act as a preventative for developing any further…if she is that self consious about them.