Coffey saw what was in men's hearts, but perhaps not what they did specifically unless making physical contact with them. We saw several instances where visions were seen or shared after physical contact was made.

When Wild Bill arrived on the mile, Coffey said "careful... careful" because he knew he was evil and dangerous while not knowing what crime or crimes he actually committed. Remember when Bill grabbed Coffey by the arm? It was at that moment that Coffey saw through a vision that it was Bill who killed the little girls that Coffey was accused of murdering.

From what we saw, Del seemed to be morally reformed, and that's what Coffey likely discerned in the short time they had together on the mile. Since Coffey and Del never had any physical contact, it would seem Coffey would not have seen through a vision what crime Del committed.