Hank had been around magic enough that he knew he had to be careful. He couldn't just waltz into the library and do research. There were many magical traps to keep less magical people ignorant but Hank already knew his way around most of the traps.
It didn't take him long to learn all about Mentor, the inanimate being. Mentor was a wood golem who was given more intelligence and personality than a typical wood golem. However, he still mostly just obeyed the instructions he was given. His instructions were to teach anyone who summoned him about inanimate intelligence. Mentor was never intentionally rude. He just never fully understood the nuances of human speech and therefore would turn anyone who summoned him inanimate whether they wanted it or not.
This wasn't new information for Hank. He never found Mentor useful because Mentor's transformations were never permanent. The duration of the transformation varied from case to case but each student would be back to normal the next day. So what Hank really wanted to know was how Justin had interfered with the spell and could Hank trick other boys into interfering with the spell in a similar manner.
Hank finally found something useful though. Justin must have used the Theta Wave Counterspell. This counterspell requires concentration that puts the brain into a sleep like state but not fully asleep. This state on the edge between wakefullness and sleep causes an increase in wish magic, that is wishes are more likely to come true if they are made while in between sleep and wakefulness. It is not a safe way to do magic as the wishes are often distorted so the Theta Wave Counterspell is only taught as a last resort, a counterspell only to be used if no other counterspell will work. Justin himself was a perfect example of how the counterspell can twist and distort instead of giving Justin what he wished for.
This information was useful because if anyone summoned the Mentor even accidentally while in between sleep and wakefulness or while sleepwalking or while dreaming, then the probability was high that the person would use the Theta Wave Counterspell by reflex. So now all Hank had to do was decide who he was going to target and he was going to trick that person into summoning the Mentor while in just the right mental state to mess it up.