I would tell him that unfortunately there are some mistakes in life that you cannot make restitution for no matter how much you wanted, killing his own wife in front of his children is beyond anything unimaginable. I won't advise him really, I would say he is lucky he did not get the death penalty which I think he deserves based on the crime committed.
I don't know what I would say. Probably after asking him what he was thinking, not thinking I would tell him to try to live out his days in constant repentance so that he can have some kind of hope for the next life. We could all do that, repent I mean but in his case its dire.