1 - Make sure they are cleared by their Doctor before starting to train
2 - Start light & gradually increase the amount of weight
3 - Keep the encouragement high & make sure they double up on water + healthy foods

Can exercise affect your breast milk if you are breastfeeding?

For the first several weeks after giving birth, your exercises won't entail jumping, running, weightlifting or anything rigorous. But once you do get to a healthy fitness level where your workouts really get you moving, you may be wondering how exercise affects breastfeeding.

"There's a misconception that you should breastfeed or pump before your workout, as high-intensity exercise can cause lactic acid buildup in your breast milk, which babies won't eat. This is probably very rare. That being said, I do still recommend that you pump before a workout simply because exercising when your breasts are full can be uncomfortable," explains Dr. Mundy.

Here are some do's and don'ts of exercising while you're breastfeeding:

Do drink plenty of water
Do wear a supportive bra that fits properly (not too tight, but not too loose)
Do gradually increase your activity level
Don't exercise with full breasts (breastfeed or pump before a workout)
Don't overdo it — stress and fatigue can reduce your breast milk and increase your risk of a breast infection

https://www.houstonmethodist.org/blog/articles/2021/feb/postpartum-exercise-what-to-know-about-exercising-after-pregnancy/