Its ultimately her choice and if you tell her to stop or you maje her feel like she is not being a good mother already you are guaranteed to lose your friendship. I smoked when needed in my first trimester because nothing helped not even the meds that were prescribed to me for my morning sickness. Its also a decision I made with my doctor after trying multiple medications. Its her decision and I honestly that there is no reason to worry about what someone else is doing in their pregnancy
Although I personally would not do it because of the possible risks, smoking a small amount of weed during pregnancy isn't super harmful. As long as she's not smoking a ton everyday the baby will probably be fine. I was very young with NY first pregnancy, and smoked occasion aly. My child is perfectly normal and healthy. She is very smart. Still I wouldn't do it again, but there are much worse things she could be doing.