Many would tell you to move on immediately; I disagree at a cursory glance - hear me out. What matters the most is why she's doing it. Everybody lies. Everybody has skeletons in their closets. It's entirely possible that she's extremely attracted to you on both a physical and emotional level (personality trumps looks most of the time for women). Therefore, she could merely be insecure about her own looks and is seeking approval from other men. Knowing you wouldn't approve, she had to do it in secret. If this is the case, perhaps you should try developing a more open relationship by expressing understanding and trying to build her self-confidence up by complimenting her more often. I would not write someone off merely for being insecure and unwilling to communicate about it.