[EdZ]

His grip might have been "stronger" on the club but his left wrist was still just as flat as it is now at the top of his swing.

Yes, the key is the left thumb position in either case. Right now his left thumb is not in a position to provide the most 'support' at impact and he must compensate for that with his right side and a slight left wedge breakdown (or wrist arch). When he doesn't - fore right. The right side being a touch too high (too much on top of grip, forcing right forearm too high) doesn't help that either. Combined they require a very full roll to square up. I would tell him to either adjust his grip slightly, or hit a full roll fade from his current position.