What if Joey really hadn't had any nefarious motivation for sending Henry to the workshop? It occurs to me that the music department where the elevator is is probably only accessible via staircase. And Joey's in a wheelchair. What if he just literally made Boris, and was like "I gotta show Henry!" And he leaves, goes home to call him.
Joey knew that Susie turned into a repulsing, shapeless slug. He might not have known she evolved to butcher angel afterwards.
I'm not saying he's innocent. Just an idiot. I'd laugh my but off if he really just did send his best friend over to see a living toon, never realizing that the monsters he'd locked far below, whom he probably hadn't thought of and assumed long dead, would come to take their revenge on him by kidnapping Henry.