Having taken some flak here for my reaction to that line, I'd clarify that I don't blame the writer for feeling that way. That's perfectly natural. My critique is that the injection of a summary opinion cheapened the writing and flattened the complexity. In a literary sense, the author was doing a great job of showing, rather telling the reader what to think, up to that point. The reader may very well have drawn the conclusion that the father was a coward. In a legal sense, the flash of bitterness actually harms their case. It draws into question their reliability as a witness. Calling it a "slip" was insightful, as it implies both.