A Mind Flayer hits its target with an attack using its tentacles. The target, a barbarian, becomes grappled, and also fails the Int save and becomes stunned:
Tentacles. Melee Weapon Attack: +7 to hit, reach 5 ft., one creature. Hit: 15 (2d10 + 4) psychic damage. If the target is Medium or smaller, it is grappled (escape DC 15) and must succeed on a DC 15 Intelligence saving throw or be stunned until this grapple ends.
Then, on its next turn, the mind flayer can use the Extract Brain action on the barbarian:
Extract Brain. Melee Weapon Attack: +7 to hit, reach 5 ft., one incapacitated humanoid grappled by the mind flayer. Hit: The target takes 55 (10d10) piercing damage. If this damage reduces the target to 0 hit points, the mind flayer kills the target by extracting and devouring its brain.
Can the barbarian do anything on his turn to break the grapple before this happens?
Is the barbarian stuck? What kind of action could another character do to break this? I had another player character cast polymorph, but the Mind Flayer's Magic Resistance trait prevented it from working. If the MF gets another turn in combat, our Barbarian is likely dead, permanently.
I'd like to know the options my players could take, even if they don't know themselves.