Is tourette's syndrome a mental illness?

Okay. I know that it isn't, but I don't know how to explain to someone that TS is not a mental disorder, but that it is a neurological disorder. This person seems to think that all brain conditions are mental disorders. I don't know how else to explain that TS is not a mental disorder. It is bugging me that he won't understand and keeps thinking that I am wrong. He is also extremely disrespectful and I want him to shut up already.