E.K.
Yuck -- I'd say stay out of it. But...
Am I understanding you right? Does the Mark neighbor really not know that he replaced ladders that were not stolen? And that in fact, Derek now has 2 sets of ladders? If I am understanding this right, then a police report has been filed that has erroneous information. And if that is the case, you can call as a dutiful citizen and report what you know about the "theft" to the police and they will handle things with Mark-n-Derek.