What action can be taken if being sued for defamation of character and slander?

My father is in the construction buisness. He has been doing a job for a person since before thanksgiving of 2010. It is a big project requiring alot of materials. This person has not payed him for his work as of yet today. My father had to put a lean on the building so that this person would pay him his money so he could get the materials to finish the job. He went to the owner of the residence to tell them the reason he put the lean on the building was because the person had not yet paid him for he job.The person was outraged and is now threatining to sue my father for defamation. What action can be done?