The thing is, the world of Shadowrun doesn't have a lot of baked in way for something to not be available to the players if its available to an NPC. For Technomancers, it's pretty much just Dissonant stuff or something prohibitively expensive.
But, if you're going with a "slightly insane" angle: he's Dissonant (Dissonant Technomancers are insane because they're Dissonant, rather than the other way around). He didn't find a Resonance Realm, per se, but some sort of Dissonance whatever. Which also handily gets around doing a Resonance Realms Search into the Archive to find the thing.
And while a GM gets to break the rules, he should always be able to justify it, not just do it because he thinks its cool - that's a good justification for an idea, but the world needs to be consistent, and the rules are part of that.