Even with half of the force of Yoze's Death Finger being reduced before hitting its target, the amount was still plenty to explode Geo's heart. Unfortunately for Geo, even though he reacted fast and quickly mobilized his crystal energy, the sphere he created simply wasn't strong enough.
If he had more time to react, he could have used more of his crystal energy to form a perfect sphere around his heart that even Yoze couldn't destroy. If he wanted to, he could even modify one of his protection spells to exclusively protect his organs.
Yet, this was the real world, and what if and what was possible didn't play a role in changing reality. So even though Geo did his best, Death Finger's force shattered the force field surrounding Geo's heart, only losing another tenth of its strength.