I'm guessing that because this is CyLynn you're not so much talking about a pre-negotiated CNC scene and more about how Cy is with Lynn, where it's more like actual rape. In that case Isabelle would be...devastated, to say the absolute fucking least. The sense of total betrayal and violation would activate a lot of her old thought patterns and in the ensuing meltdown she'd blame herself for it and hate herself for repeatedly attracting predators, which flows into even more self destructive behavior. I do think she would distance herself from Cy completely, but that would probably only lead to her looking for another sadist...preferably a more blatantly abusive one so at least this time she'll see it coming.