If the messed up man is disposed to the faith, then supernatural revelation and the supernatural means of virtue are the first thing I should discuss with him. If he is indisposed to the faith, then, yes, I could try to lead him from where he is a bit further.
However, all the while, I should know that he is setting his life’s compass by some final good other than the Christian God, the only true God. And if he is setting his life’s compass in this way, I should know that his life will be tragically flawed from its foundation. In short, the very “straightening out” of his life in the “natural order” is itself already touched by error and deception. And these he will need to shed.
Grace does not “build” on nature the way algebra builds on arithmetic. Algebra does not heal the iniquities of arithmetic. But grace heals the iniquity of our failures in the natural order, failures that we cannot remedy without grace. Further, grace sets the very orientation to the only end that God has set up for his – the supernatural vision and love of him. And only a life geared towards this vision can be a good life.
That is why, although Christians can and should cooperate with non-Christians in achieving realizable goods immanent to the eschaton – such as a peaceful society, orderly marriage, search for the truth, healthy technological development, life of human virtues – nevertheless, all such cooperation is marked by the greatest tragedy in that those with whom we cooperate and for whom we wish the greatest good, Life in the True God, are dead set against us on this our greatest love. Or they are lukewarm. Or ignorant. But in no case are they at one. Thus, tears should be our bread in this cooperation. Not “optimism” about how awesome natural law is.