I'd argue that's besides the point. The game is supposed to facilitate collaborative fun. An important thing to remember as a GM is to be a fan of the players. Want them to succeed. Make it challenging, yes, but never make it "DM vs players".

Besides, think of the lost dramatic potential. Perhaps the barbarian is haunted by what they briefly witnessed in the afterlife. Or, maybe, the cleric senses the dragon prepare the spell and as a boon their deity ensures the spell succeeds, but now the deity needs something in return from the barbarian, who didn't agree to any of this.

Or they could just counterspell it and "beat" their players.