Tenant Broke Lease for Pest Issue

[Rachel Solly]

I rent out my condominium. My tenant notified me she saw a cockroach and I called the building manager the following morning. The building sprayed that day, but she was not satisfied.  She found two dead bugs the following day, and one live bug the day after that.  On the fourth day, I notified the building there was still an issue. They had Terminix treat the unit immediately.  I have now received a letter from her attorney. She's vacated the premises and wants her security deposit back.  

Given that I did my due diligence and did not receive a complaint after the second treatment, I do not believe I am the party at fault.  However, I am not sure how to proceed.  I do not want to sue for the remainder of the rent, but I believe I should be able to keep her security deposit.

Are there any suggestions?