Doesn't matter what the DM rolls, beholders eye petrification is a DC only ability, it always hits, its up to the player's DEX Save to resist the effect.

8. Petrification Ray. The targeted creature must make a DC 16 Dexterity saving throw. On a failed save, the creature begins to turn to stone and is restrained. It must repeat the saving throw at the end of its next turn. On a success, the effect ends. On a failure, the creature is petrified until freed by the greater restoration spell or other magic.

https://5e.tools/bestiary/beholder-mm.html