What is the ruling on the food we ate, and do I owe anything to clear my conscience, knowing that I did not ask the restaurant owner directly, but rather through the young man who paid on my behalf?

The price of what you ate remains a debt upon you and must be paid to the restaurant to clear your conscience. You can then claim back what you paid from your friend.

What you ate is not unlawful for you, because it was taken with a price (debt) owed, not through usurpation. The sin of delaying payment falls upon your friend, whom you trusted.