Truly it depends on your genes and how healthy the mother is. I've known women who drank, smoked cigarettes and had no problem. I've known others who did the same and had a child with heart problems who later in life was mentally slow.
I personally stayed away from all cigarette smoke, alcohol, caffeine, high mercury fish and lunch meat. Why? Because prematurity and heart problems run in my family. I was in perfect shape and health. My daughter turned out 100% healthy and is very intelligent. But I *gasp* fed her formula which they say isn't as beneficial. Why? Because I started working again, and knew I wouldn't eat a perfect diet for her nutrition. She turned out great. I also still have *gasp* whole milk in the house and apple juice because I think the obsessiveness over obesity in kids is ridiculous. My whole family was underweight until they reached their 40's when their metabolism slowed down. Then they became "average" weight. I hope my daughter doesn't gain my skinny curse! Bottom line, it's all about genes. Know your genetics, and you'll know what you should and shouldn't do.
The world is full of old wives tales still, regardless of the scientific age. Studies don't factor in all vital information. Many studies are bias. Take two twins/sisters, get them both pregnant, have them eat the same things, do the study and then see what happens. That's the only way a study would be perfect.
More old wives tales! When I was a baby they said you should always put your baby on its stomach. So my mother did so with me. When my daughter was born they said to always put your baby on its back! I did neither. I put my daughter on her side only because I was afraid she's spit up and suffocate (on her back) or smother herself (on her stomach). Sometimes common sense and mothers instincts are far better than listening to these "facts" that constantly change.
However, cannibis can come through breast milk, you can't deny that, so if she decides to continue smoking, I would definitely suggest she give Similac instead. The other brands always gave my daughter tummy troubles, so I always recommend Similac. And they have a great formula for weening them off formula (when they're 1 years old) and onto milk which worked fantastically for my daughter.
This seems like a pretty concrete site on it.