Hi breastfeeding in itself is a very demanding activity (can burn 300-500 calories per day) so that in itself can help a woman lose weight naturally slowly over time. But keep in mind that hormones are involved, and for some women, they may not easily lose the weight until after they’re done breastfeeding.
You can however help her adopt a healthier diet by teaching her about eating hygiene, the VPF+C principle, and hydrating every time she breast feeds. You can help her eliminate processed foods by reading ingredient labels and avoiding the top 7 ingredients.
If she doesn’t exercise, encourage her to begin a light walking routine daily in the morning with her baby, and gradually increasing the duration and distance over time.
Sleep is essential for a new mother, so perhaps finding someone to help her with night shifts a few times per week so she can get in some good nights of sleep. But also, taking naps in the day can be just as helpful and restorative when her baby sleeps if that’s possible.
Hope that helps.