Interesting that the claim is made that the FO was wearing an O2 mask.
Hmm. In the FAA world, if one pilot exits the cockpit while the aircraft is at or above FL250, the remaining pilot must wear the 02 mask. Ref: 14 CFR 121.333(c)(3)
However, I don't think there is a similar regulation in Europe that mandates wearing the O2 mask while cruising at FL380. Maybe someone more familiar with Germanwings ops can comment?