The No.1 Job of a DM is to make a session fun
In order to be able to do this, you should figure out what player types there are, and the core motivation for anyone to play role-playing games.
Mostly, players play role-playing games opposed to computer games or board games because those are open-world and with imagination, anything is possible. Role-playing games enable out-of-the-box thinking (which is impossible in computer games and nearly impossible in board games), more realistic action than board games and acting.
If you immobilise a character and the player can do nothing but to continue failing rolls, they can’t think out-of-the-box, shine at role-playing or even be part of the action scene at all.
In this context, as others have suggested, it is the players’ faults that he can’t do something about it: He made the choice of min-maxing the character, and the clerics could have prevented this situation. But ultimately, it is your job as a DM to make the session fun, even if that encompasses overlooking and tweaking certain rules. Show the player consequences, but make sure that they can still play.
Feel free to skip the anecdotes below:
For the first RPG session ever of a party, I was chosen DM. I created a nice medieval detective plot, and the players overlooked every single hint and ended up sleeping in the room with all the explosives that should go off at night. I tried to warn them subtly, but they ignored me. So what did I do? I openly rolled the damage they took in the explosion.
Needless to say, hardly anyone survived. It took my players roughly 4 hours to create characters, and after 45 minutes of playing, I literally blew it. They were mad at me, and rightly so. Although the damage and all was according to the books, I failed to make a session fun for them.
Obviously, there needed to be consequences, but severe damage and most equipment destroyed would be even more devastating than “sorry, this is it, until next time” and wouldn’t have taken away the fun of the game. I should have rolled blind and made up the damage figure
Another time, when I was a player, we were forced to fight a basilisk that could freeze you to stone and instant-kill you if it was looking at you even with you looking the other way. We as a party came up with this brilliant plan to lure the beast into a cave, jump on top of its head and strike at its eyes.
The chances were … not exactly in our favour. But the DM (a player of the former party) learned from my mistakes and rolled blindly. He ruled that we managed to jump on top of the beast, take out on eye but failed to get at the other. One of our party was mortally wounded with a curse that would become lethal in a few weeks.
Nonetheless, we subsequently managed to escape the one eye that was left and killed the basilisk, with a high body count of NPCs. We continued as outcasts of the village and had to find a healer to heal a wound that has never before been healed.
If we had succeeded with our plan, it would feel too easy, but if the harsh reality had hit us and we had died all, it wouldn’t be fun anymore.
From these two anecdotes, I have learned that game-changing throws should always be done blindly by the DM, and it is the skill of a good DM to find fun solutions for devastating dice outcomes.
What can you learn from this? In your example, the throw wasn’t as game-changing as in my examples, but nonetheless, it prevented a player to have fun (which is bad). Periodically, that has to be expected and this is something you should discuss with your players. I suspect that this Barbarian built is only able to shine in combat, and you took away this (in the eyes of a player) single opportunity to have a heck of a time. You can do this, but the player rightfully will get angry at you.
In scenarios like these, make blind rolls to aid the player, and find other means of “punishment” for bad preparation than removing the character from all action.
Maybe the cleric’s control grew weaker and the player is only incapacitated or grappled? Maybe his sword/axe is stolen during the hold and his damage is nullified?
You as a DM decide everything, and you as a DM tell a story. The dice are only means and shouldn’t dictate an outcome.