From your question I get that player in question used Illusory Reality to make illusionary tarrasque real. However it is impossible by RAW:
When you cast an illusion spell of 1st level or higher, you can choose one inanimate, nonmagical object that is part of the illusion and make that object real.
The object can't deal damage or otherwise directly harm anyone.
You create the image of an object, a creature, or some other visible phenomenon that is no larger than a 15-foot cube.
Assuming that the tarrasque is the required size, then I would recommend talking to him out of character. If that doesn't work, then you could just have him lose control; after all, it is just a tarrasque that is resistant to charm.