My friend booked his ticket using his friend's credit card. They are leaving the country together but they are flying to two different countries. What does my friend need of proof just in case he gets stopped?
The person whose credit card was used needs to be present when the other person checks in for their first flight. Once the card has been verified at check in, there will not be any need to verify it again when returning home. If by chance they are leaving at different times, then it is possible to go by the airport early or a city ticket office for the airline and verify the credit card in advance.