Smoking weed while breastfeeding

So my friend and I got pregnant around the same time and her baby is only 4 weeks old and I just found out she smokes weed and is breastfeeding. Isn’t that bad for the baby? I don’t know if I should say something to her or not because I don’t want to tell her how to be a mom but at the same time wouldn’t the thc go to the breast milk?