I was seated in F on a BWI-LAX flight in early Feb when a gentleman boarded and demanded that I was in his seat. When the FA came over, that gentleman was actually on an ORD flight instead of the LAX flight. As he left the plane, the FA was discussing with other pax about how could this happen. It turned out the GA was just tearing off boarding passes instead of scanning them.

Here's an example of the exact opposite. A colleague flew from JNB/South Africa to IAD on SA and then UA from IAD to DEN to BIL on UA. When she went to board the last leg of the trip from DEN to BIL, the gate agent told her that her BP was not valid. UA had already accepted her BP for the flight from IAD to DEN and had checked her bags through from IAD to BIL. Eventually, they did let her on the plane, but only a minute or two before the closed the door.

Tell your friend to complain to CR in the US. India doesn't have the authority to make decisions that cost much $ to the airline. While I'm tired of being directed to India, and want to speak to someone in the US, the Indian phone agents are perhaps the politest CS agents I've ever encountered. Kind of hard to get mad!