I dunno. Even in the sense of a high lethality campaign that the BBEG would use that reaction and spell slot for denying recovery rather than defending, counterattacking or causing further harm seems petty, and not in the way that it makes for a fun BBEG.

Because of that, the dead PC's player can only sit there idly and wait for the next attempt, or they might be fully taken out of the conclusion, over an interaction they had no input in participating. It isn't really the caster PC who faces the consequences.