see Oracle v. Google.
If the reimplementation was made by perusing the original, it can be considered a derivative work subject to copyright.
Sure you could say "so and so downloaded Monit using IP address of 127.0.0.1 at this time", but I don't think that really is legally conclusive.
Unless the alleged violator actually posted public comments about "perusing" the original source code there's not a lot you could do. Unless you do discovery on all of his computers and he didn't delete the Monit source code. To go through all that trouble for a copyright violation on free, open source software seems hardly worth it.