Even if she wasn't misunderstanding and Shinobu had indeed kiss Fumi, it still wouldn't change the fact that she cheated. They were still in a relationship with each other, she should've confronted him then and there and broken up with him before fucking Yuma, but she didn't. A single kiss was enough for her to destroy everything between them while she had done way worse before, like sucking the Director's and the Managaka's dicks and letting them kiss and lick all over her body and was only stopped from going further from outside interference. She quickly moved on from that, never told Shinobu about it, only fucked him to make up for it as if that's how it works. Did she ever think of that? No. Of course not, little Miss actress always has excuses for her behaviour.
I have also said all this just a few hours ago which means we're circling the same arguments again and again.