How is a liar to be advised, and what are the legitimate (Shar'i) means for that?

Lying is one of the forbidden traits of the hypocrites. The way to advise a liar or anyone who deviates from Allah's law is with gentleness and kindness, by clarifying the religious ruling, and by combining encouragement and deterrence. It is preferable that the advice be given privately if it is directed at a specific person.