I need a clarification here. Are we talking about the Sacrament of Confession, or are we talking about shooting the breeze over a cup of coffee? The former involves the Seal of Confession, which is to say, it involves inviolable confidentiality. The later ---- is just shooting the breeze. It is not covered by sacramental norms.
So to get back to the question: how does this priest who has head the confession of a murderer, know who it is?