I don't get the part about the return. Except that the substitute flyer would need a second real (refundable) ticket for the return, which would be done just like the original segment. As far as the airline knows, based on the boarding pass submitted, the original flyer showed up for the original flight and took it.

Why in the security forum? Because the ID check is allegedly related to security. And because some of the best minds on FT post here.