How would you deal with a pathological liar?
Seek counseling for the person you believe to be a pathological liar. You may not be equipped to make the diagnosis that someone is a pathological liar. Plus, if you are correct in this diagnosis, treatment of such a condition requires professional assistance.
If you are unable to afford professional therapy, call a social worker and discuss what social programs are available in your area as nearly all states have social programs available to help with the mental health of their citizens.