I fear that someone who discovered, despite my efforts, where I will intern will send them a false rumor about me. This person can claim some familiarity with me (coinhabitant).
Because I’m an expendable unpaid intern, will the internship not bother investigating and automatically dismiss me on the basis of a false rumor?
There’s no rule for this. The business may ask you about it, if it happens. You can let them know what’s going on. Or they won’t ask you, and you get no internship, if it happens. If it happens, and you are denied the internship as a result, and you can prove who sent this info, you can sue.
But you aren’t giving any details. I expect nothing will happen. This past cohabitant just wanted to rattle you and won’t actually do anything. Given the kind of rumor likely to be used in this circumstance, the info wouldn’t be believed. But you could lose the internship because they don’t want somebody with a personal life in turmoil.
I did a lot of hiring of interns, part- and full-time employees. If I got a letter/email from a “coin habitant”, I’d ignore it. The more strident or vengeful, the less weight I would give it.
(Now, if it was over-the-top whacky, the entire office might end up laughing at its author…)