They won't tell you. Most likely they used one of the internet background engines that gives "possible known relations" links.
For example, looking up my name lists all by brothers and sisters and my parents, my ex-girlfriend (she used to live with me three houses ago), and three people I have no clue who they are or how they are linked up with me.
You want to help the slime covered, scum sucking debt collectors? Unless you have positive knowledge of where the deadbeat is, nothing is going to help them.2) To help them in any way possible with addresses, phone numbers, employers. Whatever I can do to help them
Further, communicating with them just encourages them to think you know something about the hiding deadbeat and will not stop them from furhter harassing you. Note that the FDCPA protects the deadbeat, not innocent bystanders like you.

