I think if you fired her for this that would be a much different story. Promotions are not a right and there is a reason that private companies have the discretion to put who they want in leadership positions. Any company is a collection of human beings at the end of the day, and public demonstrations of character, particularly on social media, now matter in the workforce sphere - whether that's good thing or bad thing is another discussion. If trust is broken trust is broken for you, and your company is trusting you as a manager to apply your judgment. To think one's public behavior shouldn't be included in your calculus of merit is a bit naïve. Don't shit where you eat kids.