I'm very surprised that they stopped bothering you. I got debt collector calls for years for someone who used to have my number. For a while I tried to get them to stop calling me by asking for the name of their collection agency (75% hang up) or by asking that they don't contact me in any form again (compliance with the Fair Debt Collection Practices Act). The latter works, but they are legally allowed to resell your debt so the next day I'd get a call from a different collections agency. You'll never get them all to stop calling you so it's easier to deal with the one that you know.
Overall it's been an interesting experience. As someone who grew up middle class, had a scholarship, and lives frugally I would have otherwise not realized how predatory debt collectors are. Instead I've written my representatives many times asking that they create stronger consumer protections in this area.
Edit: Oh, also they aren't legally allowed to share the collections detail of the individual, so I couldn't research his debts further without claiming to be him, which I actually considered but decided against.