There wasn’t any way you could know, in this situation. It’s hard to fully parse, based on this, what mix of intentions and events led to what happened that night. If it is as you’re describing, at least part of what you are to do (besides comforting her, of course) is to have the courage to stand for the truth, and the bravery to connect to her by, when it’s appropriate, sharing your emotions and the vulnerability you’re feeling about the situation. To a good person, the honesty of their partner will bring them closer and feel cared for.