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I'm sorry to hear about your friend's situation. Unfortunately, you can't take legal action against someone for being a liar. Fraud is only really actionable if done to obtain property or money. Sadly, people lie in relationships all the time and if people could take others to court under that kind of situation, the courts would be even more overloaded than they already are.
The only thing I could think of is that if he were improperly involving his job in his affairs...say if he used his on-duty time to see her or took her out in a squad car in violation of the rules, etc...then she could file a complaint against him with Internal Affairs. But it would have to involve on the job misconduct, and not just the fact that he is a liar in his private life.
I'm sorry I can't give you better news. The situation is awful but unfortunately there is not really any legal recourse in this type of circumstance.