I guess he'll either get through the anger part and accept that a supervisor is ultimately responsible for the results of his dept and learn from the experience ... or he will continue to believe he is the victim in this cock up and not be a supervisor for too long .
If you yourself have previous experience with supervising employees , then use that knowledge to know when the time is right to offer constructive , supportive criticism . I'm not a relationship expert , but believe that honesty and compassion would come up in a list of desirable things on most people's wish list . The mix of delivering those things is up people in that relationship , but there's nothing wrong with having compassion for someone's disappointment , while also helping them to understand areas they could have anticipated better .
Rule #1 in supervising . Trust no one , check everything your *** is ultimately going to be pinned to .