Obviously, if you introduce a third character into a story about an S and an M doing stuff like this, the third person should be a switch. She could both get bound by the Ayame and do some binding on Zen. Everybody wins.
Is almost like Aoi is conflicted between wanting to share that pleasure and stopping a possible bullying. Also if her worry about him was just about the bullying she wouldnt worry about the smell of his cum but rather feel disgusted.