User blog comment:Lex wa Kaiho/What is Lumen Histoire?/@comment-223.29.204.2-20140319113452/@comment-7418318-20140319115155

Maybe Precht told him a few hints in the hopes that Ivan would openly attack Fairy Tail and thus provide another distraction for the Magic Coulcil, so that Grimoire Hearts could gather the keys in unnoticed.

Or, considering that when Prechts left Makarov was arleady 40, maybe Ivan was already hanging around the guild before Precht passed the title of Master on to Makarov. If so, it's possible that the two might have had a careless moment and Ivan was there to overhear them mentioning it.