Yeah, that confused me, too. I can't tell what the "it" is referring to: the ex's "fate", or the ofuda she was holding, which is apparently the "wrong ofuda."
... But I don't know for what, exactly. For warding off spider-lady, I assume, but the dialogue makes it sound like the ofuda and the ex's cheating are somehow connected.
In any case, Ms. Spider prevented Ms. Ex from being trisected either by (1) holding herself back, (2) tampering with her fate some other way, or (3) tampering the ofuda which would have caused the fate if she'd left it alone.
I dunno. Chapter is great; this dialogue is confusing.