On the face of it this isn't your problem. Someone you vaguely know told you that someone else you vaguely know might have done something that is at the very least wrong, but possibly illegal. If your neighbor told you that "my former neighbor where I used to live totally [breaks a law] all the time" you wouldn't feel any obligation to phone the police.
However, there's an interesting wrinkle here that you are able to confirm that it's your code, and furthermore that you went and did that. This puts you in a position of knowing that something wrong and possibly illegal has happened. I don't know whether that puts a legal imposition on you or not. I would feel like I owed it to my former employer to at least tell them. I wouldn't spend my own money for a lawyer; that's crazy. But I would contact someone at the bank. A quick phone call or email in which you give them the GitHub repo address, and tell them what you have concluded - this is the code you wrote, with tabs changed to spaces.
Then wash your hands of it. The person you report it to may followup on it or they may not. The repo may disappear, or it may not. The former coworker, who doesn't seem to be gaining much by claiming to have written this code, since others are seeing through the ruse pretty quickly, may "never work in this town again" or may continue to lie and cheat in order to get hired, for whatever reasons. But you will have done your part. You won't have helped the theft by keeping quiet once you know.
And keep a record. Save your email and any you get in reply. Or if you call, write out notes (pen and paper or digital) about who you called, what you told them, what they said. If there is some back and forth in response, update your notes every time you talk to anyone. Put these notes somewhere safe. Should it ever happen that someone else discovers the repo and accuses you of being the leaker, liar, cheater, you will want to be able to show that you reported it when you found out, and be able to tell them who you reported it to.