It`s at least commendable that your friend has expressed some kind of regret, even if it is too late to undo what was done. It`s never too late to say sorry if it is sincere and if that can help him find some peace before he passes on it will be worthwhile. Perhaps a letter or open letter would be a cathartic and effective means of communicating his regrets.
Perhaps you read this uplifting news in recent days-
US cancer victim uses obituary to confess sins - Telegraph