This is one of the endings of the 'Extended' episode.
*** Spoiler - click to reveal ***At the end, when you have your injured friend in the final room, instead of putting them in the machine to the left, you need to put them on the pedestal above by the exit. This will open the door for you; just fly up, hit the button and GTFO. They was probably dead anyway, right...?
To get the other ending (and the other trophy), from the main menu just hit Extended then Continue. This will restart you at the end -- Just follow the instructions in the other trophy: