Except I guarantee you that some of those "pirates" were legitimate customers and all it will take is ONE person posting proof of purchase side by side with you treating them like criminals to ruin you.
If he did it right, then every one of them was a pirate. There are three states the program can be in:
1) Registered copy. Anti-pirate check is run, bug is patched, everything's good.
2) Unregistered copy. Anti-pirate check is run, program tells user to get an honest copy, bug is never hit.
3) Pirated copy. Anti-pirate check is bypassed, bug is not patched, program crashes on level 10.
Note that the only way to encounter the bug is to bypass the anti-piracy check. A legitimate customer who's had the check falsely trigger will encounter case 2, not case 3.