User blog comment:FairyRaven/Fairy Tail Chapter 423: Do It All for Love, Review and Commentary/@comment-103.14.62.54-20150309110452/@comment-7418318-20150310165445

I don't think he was really in Orochi Fin.

I doubt we'll ever learn for sure, but my theory is that they hired him (either not knowing or not caring that he's a Dark Mage and wanted criminal... or caring and finding it good) to train them (they did call him Sensei after all) and gave him a bonus to act as their trump card if Lamia Scale gives them more trouble than they can handle on their own.