Traveler makes a great point on validation. Validation is not agreeing or disagreeing, it is recognizing how she feels! I feel the anti-validating sentiment sometimes misses this point. You are not validating that she is leaving/cheating/lying/breaking vows/being disrespectful. You are validating her feelings. But the reason validation is so effective is because the alternative, if you must respond, is to get defensive and argue. Between the two choices, validation is best.

If and when she starts behaving poorly and disrespectfully simply state that you refuse to be spoken to that way, then walk away.