This is not advice for a typical DM though. If she’s a teenager (or as close to it as possible) trying to run a game for the first time then I don’t think he does need to know the rules in advance, or to have his expectations managed. That isn’t something you say to a young adult doing something high pressured for the first time.
Don’t go in with any expectations, just provide 200% encouragement. If you miss every attack, so be it. If you can’t heal, do something else. If your character dies… keep smiling and roll another one. Teaching someone else is not about them entertaining you… though you might well be entertained along the way.