The IRO on that long flight is still required to file an ASAP report should an incident occur, and is still on the hook to the FAA as a member of that crew. He's listed in the flight log as crew, and on the flight release. During the course of any given flight, he may sit in the IRO seat for the duration, or may spend much of the trip in the left, and or right seat. Depending on the operator, a SIC may be along on the trip for legality, without much in the way of actual responsibility. Or he may be occupied much of the time. He is still a member of that crew, performing flight duty, and that block time will count against his flight time limitations. He is legally a part of that crew, and acting as a required crew member under the regulations under which the flight is operated. If a pilot gets up for a break and is relieved by another, the former isn't suddenly deadheading. He's still part of the crew, assigned by the certificate holder to duty as a crew member.
It's hardly parker pen experience.