@lexy
I think at least more than your average guy. Generally speaking, it's a field that non-professionals don't even touch. It's quite a specific one, too, since financially literate people just don't get involved with such places.
There are a few plausible situations:
The author-san worked as a finance worker handling lost causes for a bank or security agency, or at least had a close talkative friend that did this.
The author-san was interested in the topic and found out that it's a niche one. So, he used google to learn all these things.
The author-san used to be a loan shark or him being damaged by them is not really likely, since he wouldn't want to talk about this all that much, and in the latter case he would probably just feel remorse.
If you ask me, I'd say he was a bank worker that handled these cases. That would explain deep focus on the victims and their circumstances, since he had to learn this things to handle this people, and to tell the bank whether they can get the money back somehow. And then he just inserts these ways in Ushijima, a financially literate loan shark, something unrealistic, since financially educated people won't touch this shady stuff.