I don't get it - Spoiler Alert
When Tom finds the photograph of Molly, why doesn't Jack just play along? He's the one who was telling Molly how hard life is on the front. I would assume he'd want to let his brother go on believing he has a reason to live then break the news to him after the war or when he gets a chance to break it to him gently (assuming that's even possible). The way it's done in the movie seems absolutely cruel and, I believe, it's what leads to his distraction and death.
Woman, man! That's the way it should be Tarzan. [Tarzan and his mate]