What if you go to lunch, and you are at an Idine restaurant. Your friends gives you cash for their share, and you use your credit card because you are getting 10 miles per dollar.

Or, if instead of earning miles from Idine, you get 20% back in cash.

I see this as a bigger ethical problem than the others - in the ones involving a friend who is going to let you make many purchases you have a friend who knowingly is costing himself a lot of money so you can get miles. And I don't expect that too many friends like that exist.

In the Idine case, though, you are intentionally benefitting from your friend - in effect letting them pay more than their share of the bill.

Is this ethical, or should you feel obligated to tell them and ask if it is okay, or should you pay more than your share since it will balance out later?