Okay, well what you ought to have done is, when you are first discussing how the role works with Watson, is make sure she fully understands the role and how it works. After that point, that should be where your involvement ends, because you've explained what the player needs to know and you come away from that confident she understands it.
I would suggest that, even if she later appears to misunderstand it, you do not revisit the conversation to double check. You've already fulfilled your hosting requirement by sending the role PM and explaining it, anything further than that runs the risk of overstepping into the game. I'm given to understand that you were advised by some in the dead chat to explain it - it appears that there was some level of miscommunication here, and also in future reference I would advise reaching out to someone you trust understands Mafia that isn't involved in the game to check if you're unsure. Most players will be fair and impartial, but doing the above prevents any sort of accusation of this nature. It's about avoiding the risk of interference, basically.
To summarise, I feel like as a host, you ought to have shown more diligence in ensuring your player understands their role when you're initially having the discussion with them, and that you probably should have avoided going back to the discussion later on, especially under the advice of a player who potentially stands to benefit from this (and this is not to imply anything untoward). I would suggest having a period of time before the game starts actually, wherein you can field questions about roles from players, after which you can safely operate under the assumption that they all understand their role.