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.
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