Not making "The Face" is hard. I really don't think that it comes from any deep competitive desire to win, though, I think it comes from the shock and fear of watching someone you care about, who knows less about pool, line up and go for a shot that's totally wrong or impossible. OR watching them go for the right shot but in the wrong way completely.
Beginning/ weaker players have an almost imaginary, magical sense of pool. Given any layout they may not see all of the choices and opportunities available, and THEN they won't make the best (or even any of the top 3 best) decisions available. They'll go for something fantastic (as in fantasy), or they'll completely misgauge the reaction of the cueball after the shot.
So instead of experiencing the fear and expectation of the upcoming or past bad shot, try really hard to remember the exact layout (write it down if she won't notice) and another day, when you two are practicing together, set up the same layouts and walk her through some of the choices.
Don't be authoritative. Set up the layout again and let her make decisions about that shot and its alternatives. Try them out. Scientifically. After two or three of her guesses, gently suggest what you would do, and practice that. It will give her a better working knowledge of what she sees, what you see, and may hit paydirt into any misconceptions she's got that leads to bad decisions.