TELL YOUR WIFE. You are under no obligation to keep this woman's secret. In fact, she is putting a wedge between you and your wife by asking you to do so. If she really cared about her friendship with your wife, she would have kept her mouth shut and not asked you to cheat. TELL YOUR WIFE, THE SOONER THE BETTER, and DO NOT ever meet with this woman alone again...she is poison -- and she is putting your marriage and family at risk.