And heeere comes the backstabbing "friend" mob. With the way things are looking, all this manga would need to be like Mone-san no Majimesugiru Tsukiaikata is an MC that's actually loyal, a near-complete derailment of the story, and a horde of degenerates slobbering over the mobs the story was derailed for.
That said, it's no surprise Yukino started wondering that about Shin. The only way he could truly get bored of their roleplaying is if he's gotten bored of her--and he's already been fooling around with the other mobs. Such actions are implicitly telling Yukino, in essence, that she can't keep that pathetic guy's attention.
Like I've said, Shin does not deserve Yukino.