Can a Mind Flayer tentacle-grapple be broken by the grapplee?

I currently have a Mind Flayer that has successfully used its tentacle attack on a target.The target is a barbarian that is currently Grappled, as well as stunned since he failed the Int Save.

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 the next turn it makes the Extract Brain action:

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.

On the players turn, can he do nothing?

Stunned make them Incapacitated which means they can’t take actions (Escape Grapple is an action). Also, Stunned means they automatically fail STR/DEX saves.

Is the player stuck? What kind of action could another player do to break this? I had another player cast polymorph, but the Mind Flayer’s magic resistance 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.