It would also depend on what type of delivery she had. A c-section vs a vaginal birth effect the rectus abdominal differently.
I would first train with stabilization of the entire core. Much of this area changes during pregnancy and then afterward since the client is doing a lot more sitting for breastfeeding, changing diapers and holding a cranky, sleeping infant. Focusing on proper alignment while sitting and standing will also help the mom become more aware of their posture and how that affects their musculature.