He probably just sold it to a friend so I really wouldnt stress about it and no I really dont see it as crossing the line. If he was selling mass amounts then that wold be another story. There is really only so much a parent can do for their children and when they try to do too much it oftens backfires.
The problem her son probably has is seeing that his mother smokes and that he sees her being a hypocrite. The best advice I can really give is just to have a loving conversation and NOT to treat him like a child when doing so. He's going to continue smoking so its something she's probably just going to have to deal with