What should the lessor do when lessee sublets against the lease he signed?

When a lessee sublets against the terms of the lease agreement, the lessor should first review the lease to confirm that subletting is prohibited. If it is, the lessor can issue a formal notice to the lessee, demanding compliance with the lease terms and potentially seeking remedies for breach of contract. This may include terminating the lease or pursuing legal action if the lessee does not remedy the situation. It's advisable for the lessor to document all communications and actions taken regarding the violation.