User blog comment:Relaxing94/Natsu vs Lucy (sooner or later)/@comment-197.226.179.22-20140822141317/@comment-7418318-20140822215821

Makarov wondered how Ivan could know even the name of Lumen Histoire, so he clearly never told him. After wondering that out loud, Mavis said that it was most likely Precht who told Ivan about LH.

I can't tell you the exact chatper/episode but it's shortly after Laxus beats the crap out of Raven Tail with the scene ending with Mavis crying and Makarov telling Laxus to comfort her.