Atonement who was that guy?

The French guy? He was just a patient whom Sister Drummond asked Briony to speak with because he was dying. He most likely died from the gruesome head injury, which also affected his brain. That's why he was talking to Briony as though they were longtime friends. I think he was used as a catalyst to Briony's maturation. She saw the suffering of the soldier and how Robbie, too, must be suffering, and that she was the one who caused his distress. It's a beautiful story, and I suggest reading the book by Ian McEwan to fully grasp the character developments.