Given the information, the guy had every right to take that bike. If it was on a public street (or on someone's property it doesn't belong to), not locked up, in as dumpy condition as he makes it out to be, the original owner probably doesn't want it. What if it was a busted television? Or maybe an old refrigerator? A skateboard? An old razor scooter?

And OP, you should post pictures of it.