User blog comment:IamJakuhoRaikoben/Chapter 449: Mavis & Zeref, Review./@comment-26389992-20150826010359/@comment-9595671-20150827235229

Yeah, Zero showed just how smart Mavis can be other than as a Tactitian. It seems more likely to me when he taught the group magic he didn't realize he was teaching Mavis enough she found out how to cast Law.

Considering how it works and how he would have had to test it, I doubt he physically showed and taught Mavis how to use Law. Considering this is when he was loving life before Hades ruined it and he cried when she ran off, he probably just treated it as another sin.