During the last game of D&D 5E I DMed, my party containing a level 5 Wild Magic Sorcerer encountered a Beholder Zombie. During this fight a Wild Magic Surge effect turned him into a potted plant. Fortunately for him the beholder did not get the chance to use its Disintegration Ray on him but I was wondering what would have happened if it did? (Howerver the beholder did land an Enervation Ray that dropped my player unconcious right away)
First of all would he have been able to save against it? The PHB tells us that turning into a potted plant incapacitates him and make him vulnerable to all types of damages but does that mean that he could still move while turned into a plant? On the spot I ruled it as Paralysis not allowing him to move and allowing him to make a Consitution Saving throw against the Enervation Ray but reading back the PHB made me wonder if I made the right decision.
Secondly, if he did get hit by the Disintegration Ray (and went down below 0 hitpoint), would he have been disintegrated or would the potted plant form would have been disintegrated? Since both effect take place when he goes down to 0 hitpoints I don't really know which effect to apply first. Since it would totally sentence the character to permanent death at this level I wouldn't like to make my player reroll a character after literally getting one shotted by an enemy within its capabilities if there is a chance he did not actually die.