14 CFR 121.333 C (II)
"(3) Notwithstanding paragraph (c)(2) of this section, if for any reason at any time it is necessary for one pilot to leave his station at the controls of the airplane when operating at flight altitudes above flight level 250, the remaining pilot at the controls shall put on and use his oxygen mask until the other pilot has returned to his duty station."
Quotes like that do not help at all. Remember that the aircraft was not flying according to FARs and that rules regarding oxygen use are very much different in euroland. European airlines simply do not fly according to FAR 121, that is a local rule in the USA, whereas in europe a multinational approach is used under the legislative rulework of the EASA.