CLS, you are right, it is a tough one for the skipper as it is a technically legal crew on paper, but is it safe? Funnily enough it might not have been legal before the blanket 18 hrs for ULH was surreptitiously introduced as everyone had to earn their FDP extension with the appropriate in flight rest. This sometimes meant burning out the Relief who now, surprise, surprise is doing the landing as well. The whole practice is a fiddle and should be banned except maybe used like the 3 pilot option to get the aircraft home
In the meantime if it was up to me: If the relief F/O was under 12 months in that position OR was on the same time zone as the Capt, (ie. either capt or F/O would be landing unrested off his/her local time), the duty would be unsafe. Enough cancellations would get some attention I’m sure.
Just a thought.