This is a hard one.If i was in your shoes i would ring her up and ask her did she know your step son is smoking?
Hopefully if you tell her about this she can try not to incourage it.
It sounds like she is the reason he started and she needs to be dealt with.
I hope she isn't into other drugs because it could fall back on him.
I would also have a chat to your step son and ask him what's going on why has he started smoking.
Like you said you can't stop him but tell him this has to stop now.
Goodluck i hope you can work it out nicely