He probably got a letter pointing out the obvious: the DTK is legally Apple's property. You don't pay $500 to own it, you pay $500 to rent it, and the terms and conditions (available from Apple's developer website without even applying for the chance to rent a DTK) make it very clear that the loan is personal. As in, outside of fairly narrow approved scenarios, you aren't even allowed to loan out the unit.
For example, an employer can shuffle their DTKs between different employees working on porting software to ARM as needed. Handing one off to an outsider who has no intent of working on porting Mac software to ARM, however, is right out.
So all Apple had to do was let it be known that they'd have police knocking on LTT's door to recover their stolen property and possibly arrest someone, and that the dev who supplied it to LTT would face consequences, and I'm sure LTT rolled over.