Smoking while pregnant

Let me start by saying this isn't about me, so haters can leave now. My friend is a few weeks further along than I am and still smokes cigarettes. So far the baby is growing as he should and there doesn't seem to be any ill effects from it, but it's always been my belief that you shouldn't smoke while pregnant. Being a smoker myself I know it's hard to quit, but it's not just you smoking once you're pregnant in my opinion. My question is this: should I keep my opinion to myself as its her baby, not mine, or say something or intervene more aggressively (she hasn't told her OB that she's smoking)?