Ask after they lose

Right after a loss is the worst time for advice and the best time for one question, asked gently, later: "Which moment did the game turn?"

Let them find the moment. If they cannot, you can play the game back together and stop at each move: "Was it still fine here?" They will usually see it before you do.

Never list everything they did wrong. One moment, found by them, is worth more than ten found by you.