They either trade ryze or trade xayah. If they ban only xayah, they leave ryze for G2, and if they ban only ryze, they leave xayah for g2. They either ban both or none. None of this situations are good for FPX, since ryze can be countered.
If G2 takes Ryze that's good for FPX for multiple reasons. First of all neither Wunder or Caps is nearly as good on Ryze as Doinb. Second of all getting Gragas means Perkz will have a much harder time playing Yasuo.