The lawful use of the law is to show someone BY THE SCRIPTURES that they need the Saviour. It certainly is not to show anyone that justification and/or righteousness come by it (Galatians 2:16 KJV. Galatians 2:21 KJV). To place yourself under it is to fall from grace (Galatians 5:4 KJV). Now, get on with the ministry of reconciliation! Show someone how that God was in Christ, reconciling the world unto Himself, not imputing their trespasses unto them (2 Corinthians 5:19 KJV, 2 Corinthians 5:21 KJV). Tell them how to be saved and sealed (1 Corinthians 15:1-4 KJV, Ephesians 1:13 KJV, Ephesians 2:8-9 KJV)!
The question is if unbelievers should be taught to obey God or if this is only for believers. Salvation is not by works is not disputed.