Pathological lying is not a formal psychiatric condition and indeed most psychiatric organisations have yet to provide it with a strict classification. It's often a symptom of other conditions, such as narcissistic personality disorder or antisocial personality disorder, but the particulars often vary from case to case. Helping a pathological liar may be extremely difficult: the liar needs to seek help on his own and might not be willing to admit he has a problem. If you want to provide help, you need to understand both the extent of the condition and the limits of your ability to intervene.
Look for symptoms of pathological lying. Unlike normal lies, they rarely serve any tangible benefit except perhaps to impress others. They're often grandiose, self-serving and extremely complex. They take place regularly (someone can tell the odd whopper or two once in a while without being a pathological liar) and they're usually delivered with no concern about how they affect other people.
Consider whether the lies are part of some larger psychological condition. If the liar exhibits signs such as wild mood swings, impulsive behaviour, routine disregard for other people's feelings or similar instabilities, it can suggest that the problem goes deeper than compulsive deception. It needs to be treated on a comprehensive level before the lies will diminish.
Draw firm boundaries between your own emotional health and the liar's. You can't help him if you're confused by his deceit or willing to buy into it on any level. It can be difficult sometimes, because pathological liars often mix their lies with bits of the truth. Take everything he says with a grain of salt and refuse to play along with anything that might cause you damage (such as loaning him money or accepting his word about another person's behaviour). Never "cover" for a pathological liar or make excuses for his behaviour, especially if they could damage you.
Prepare proof about the liar's fabrications and be prepared to exhibit it if necessary. Confronting the liar directly is likely to make him defensive, but gently pointing out the trouble caused by his lies may induce him to seek help.
Speak to the liar about his problem and use him to get help. Keep your temper, speak calmly and rationally, and don't allow yourself to be goaded. Tell him that his lies are causing difficulties and cite proof if you need to, but don't take on any smug or triumphant tones. If you're a friend or a loved one, present yourself as someone who cares about the liar's well-being and wants him to get better. Don't allow the liar to side track you or take advantage of your sympathies. Remember, he may have a tough time acknowledging his problem, and you can't force him anywhere he doesn't want to go.
If the lair appears willing to acknowledge his problem and accept help, ask him if you can contact a qualified therapist and set up an appointment on his behalf. It will be harder for him to put off making the appointment if you do it for him. Make sure that the liar knows and accepts your intervention, however, and that the therapist is clear about the situation. Don't do this more than once. With a single appointment made, it's absolutely up to the liar to continue to seek treatment.
Pathological liars can cause a great deal of damage to people around them, especially if their lying is part of a larger psychological condition. Understand that the lying is not your problem and while you can be helpful if he genuinely wants to change, you can't change for him. In the event his deceit becomes too painful or harmful to you, be prepared to end the relationship and break off all contact.