Dear forum members,
I am trying to help a close friend who had an issue with a flight to HongKong on June 2nd. He was booked on a Lufthansa ticket (220 ticket stock) in C from Stockholm via Copenhagen and Frankfurt to HongKong, first two legs on SK (SK407, SK1635/LH6225 operated by SAS), then Lufthansa LH796.
First flight SK407 was only slightly delayed, so that was not an issue, but flight SK1635 was severely delayed, so he was automatically rebooked to LX1273 from Copenhagen to Zurich and LX 138 from Zuruch to HongKong the same evening, only inflicting an arrival delay of 1hour 20 minutes.
However, that LX 1273 flight was also delayed by about one hour and since the flight to HongKong was not waiting in Zurich, he had to stay overnight in Zurich. He was provided with a hotel room in Zurich and rebooked for the next day from Zurich via Frankfurt to HongKong, (again on LH796). Since he had an appointment in HongKong he could negotiate in Frankfurt to get rebooked already on the earlier Cathay Pacific flight CX288, yet arriving in HongKong more than 16 hours later than scheduled.
He submitted an EC261 claim, first with Swiss, because the large delay was inflicted by them, but they referred to SAS as the responsible airline for the delay. He then also submitted a claim to SAS, but after some wait he received a negative reply stating that SK1635 was delayed due to an "industrial action initiated by the Air traffic restriction. As such, SAS could not operate flight SK1635 / 02 June 2023 according to the published schedule for reason of Air traffic restriction, a situation explicitly mentioned as an extraordinary circumstance in EU regulation 261/2004 as referred to in Article 5 of the regulation and expended in recital (14) of its preamble."

As I have not encountered such situation myself, nor can I really judge whether he should just drop his claim or rather re-iterate the claim with Swiss airlines, I thought somebody here in the forum could help me out with a suggestion.
Many thanks in advance!