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If the collection agency sues the tenant, then the tenant can certainly defend himself by arguing that the apartment was uninhabitable. The biggest problem in this situation is that it is up to the tenant to prove that the apartment was uninhabitable. If he has no pictures or witness testimony, then it'll be his word against the landlord's word. The judge may or may not believe him. However, if the judge does believe him, then he would likely win the lawsuit.
If the collection agency does not sue him, but simply destroys his credit, then the tenant can sue the landlord and collection agency, and make his legal arguments regarding the apartment at that trial.
Does that answer your question? Please let me know if you need clarification, as I am happy to continue helping you until you are satisfied.