He was a middle office worker, in security/admin, where he verified then authorized big trades. He used his excel and VB skills to impress a front office manager and get a place in the front office as a market maker (I think he has been assistant analyst in between the two job, but that isn't relevant).

Société Générale used the same id / email adresses for him after he changed jobs, and since the user management was lacking (and wasn't audited), he was allowed to make huge trades as a MM, and authorize his own trade as an ex-security/admin guy.

I might be wrong about his exact job title in the middle office, but I 100% guaranty you the rest.