Author here. We do not know for sure how they unlocked it, but the phone was locked with a numerical pin. My guess is that those numbers must have been easy to see on the screen based on the smudges alone. I wanted to ask for more details, but I decided against further traumatizing him with my (as far as he's concerned) pointless geeky questions.

Also, trivia for iPhone users: my brother used to have Face ID set up, but he disabled it because he couldn't figure out how to set up a second face and it was annoying when he needed to share the phone with his wife. So don't do that!