Well, first I'd have to know the person had Tourette's. Then I'd have to believe that they were being truthful about it, and not just taking the piss (far more likely, statistically). If I knew they had Tourette's, I'd be polite, of course. But I'm afraid I wouldn't seek to be around them any more than I had to be, as I don't enjoy being verbally abused.
People also never seem to think about the fact that the person being abused also has their own issues and struggles, and being called a slur might just be the last straw. There's no consideration for the other person.
What if, for instance, a person with Tourette's calls a woman recovering from an eating disorder, or even just one struggling with insecurity and low self-esteem, a 'fat cow' repeatedly? And even knowing it's Tourette's, that tic exacerbates her feelings of dysmorphia or self-disgust, and causes mental health issues for her?
I don't know - it's difficult to balance. But I do know that we can't just let people do whatever they want because they can't help it. Is verbal abuse over that line where someone's actions need to be curtailed? I don't think so. But equally I don't think you can expect someone to like a person flinging applicable insults at them, which might cut to the quick.