You can find Lilly Millet at Emerald Ranch, northeast of camp.
Specifically, she’s sitting outside a large building next to a man named Cooper… who will start fighting you after trying to get the money from Lilly. Take the time to put on a disguise, then beat him up to find Lilly’s debt money.
If someone sees you do this, they will assume you’re stealing despite Arthur’s audible protests that he was just collecting a debt. Thus, the disguise. Remember to turn the money in at camp.