Untermiete not leaving?

What happens if a Untermiete decides to not leave the apartment? Hauptmieter cancelled the lease (without a valuable reason), gave a 3 months notice period. Even though there was a possibility of a take over, the renting company refused (they never asked for documents). New apartment is still not found and Untermiete wants to stay for the next month or two paying the rent, until he finds a new apartment.

Is it possible?