Can you smoke and drink when breastfeeding?

My cousin breast feeds and when I was visiting her, we smoked a lot of cigs and drank beer or wine a lot, she even mentioned doing Molly (I do not mess with drugs, I've actually quit smoking too, I just drink). I told my bf and he was so upset, telling me how stupid she was for risking harming the baby, I feel dumb for not even thinking about that! After he said that I remembered how she said her baby was in the very low percentile for body weight and height, etc. Do you all think drinking and smoking affects the breast milk? Also it's kind of annoying because my family thinks I'm the problem child because I drink beer and live with my bf outside of marriage, but they have no idea that she does this, and that she sometimes does drugs with the baby's dad (who she's not with). 😐