Another one of Mare's high-school friend, Beth's brother Freddie has an opioid addiction. To support his habit, he's stolen from his sister numerous times and even gone as far as to catfish Katie's mother Dawn by falsely pretending to be the one holding Katie captive and asking for ransom. While I doubt he would've been able to execute it all flawlessly given his physical state, he might've seen something that could be important to the investigation.