It's libel. He libeled you. Slander is oral. Tossing around legal terms is slippery business.

Now, that said…..

Before this all turns south and becomes a flamewar over who stole what from whom, just don't go there. You've both said your piece. And I'm removing the needless personal attacks Tridor stuffed onto the end of the post.

The codebase in question isn't even original enough to be released from Russ Walsh's copyrights also, which makes it freeware. (not over 51% original from copyright)