Liars are tricky to deal with - you never know where you stand. Your Liar might be trying to usurp your authority, steal your job, or just get away with something you wouldn't condone. To deal most effectively with a Liar, remember:
1. Being honest yourself will not convert a Liar. Don't let yourself be exploited.
2. Take defensive action if the Liar is your co-worker.
3. Make sure your boss isn't in on the lie before going to her for help.
4. Document your case against the Liar so that it's not your opinion versus his.
5. Use your human resources department to back you up with its authority and procedures.