This is completed in Chapter 4. In case the video goes down, little bit after the beginning of the chapter after you have finished using the cannon to keep your shuttle safe, the group will discuss wearing suits to abandon the shuttle and board the Ishimura. The group will walk over and start to put on the suits. As you're putting on your suit, you look back over at Lexine who will drop her helmet and bend over. Whilst she is bent over, when hearing Gabe say 'taking your time,' move your aiming reticle/cursor over her behind and press a button to focus it. The trophy should pop soon after. If you leave the shuttle, you went too far. If you miss this opportunity, you can use the Chapter Select to re-play the level for another chance at this.
Timestamp: 5:00 - 5:58