"When Jesus perceived their thoughts, he answered them,'Why do you question in your hearts? Which is easier, to say, ‘Your sins are forgiven you,’ or to say, ‘Rise and walk’? But that you may know that the Son of Man has authority on earth to forgive sins'—he said to the man who was paralyzed—'I say to you, rise, pick up your bed and go home.' And immediately he rose up before them and picked up what he had been lying on and went home, glorifying God." (Luke 5:18-25)
This passage of Scripture has not been tainted (yet) in the New World Translation (NWT) so have them use their own bible. As you begin reading, emphasize verse 21, where it says, "Who is this who speaks blasphemies? Who can forgive sins but God alone?"
The JW's will probably try to stop here to twist what Jesus was saying or attempt to leave. Share some verses with them that DO show God forgives sin, but return to Luke 5 and continue reading the passage. It is very important to finish the passage from Luke (Mark 2:6-12 is the same account and can also be used).
When you get to verse 24, put emphasis on "the Son of Man has authority on earth to forgive sins." They should begin making the connection and a seed of doubt will be planted inside their head. Will your visitor drop to their knees and cry out to God in repentance at this point? Probably not. In fact, you probably won't notice any difference, but as they leave, pray that these misguided people will think about what you've shared with them.
Remember, you must be loving and humble in your interactions with them. "Know this, my beloved brothers: let every person be quick to hear, slow to speak, slow to anger; for the anger of man does not produce the righteousness of God." (James 1:19-20). Your actions will speak louder than your words. Try putting yourself into their shoes and imagine how you would want someone to correct your beliefs.
OTHER HELPFUL VERSES:
Psalm 103:2, 3
Psalm 130:3, 4