Out of all the things Sou could have done, this is the most Sou... and most damaging to Yuri, and there's no winning for Yuri.
If she builds up Sou tolerance, then she loses because she's still using the "drug" that is Sou. If she doesn't build up Sou tolerance, she also loses and gets mind broken.
Sou is a fucking menace
Also, Classmate A's powerup from the Hugpocalypse obliterated by Yuri's Locker Scenario in an instant.
I would also not be surprised if that locker regularly starts getting visits by the other girls so they can absorb Sou's boysmell.