could there be legal problems (domain forwarding)?

a friend runs a website in his country, which is legal by law.

now he ask me if he run in his country a premium domain like cars.fr and in another country are cars not legal but he can buy the domain cars.us
could there be a problem if he only sets up a redirect from cars.us --> cars.fr?

many people enter the URL directly in the browser so he could get additional traffic without actively advertising.

just google it, man! If nobody's answering, google it

Personally, I think you'd only run into troubles if the word "cars" (as per your example) was still trademarked in either of the 2 countries (FR / US), but otherwise I don't think there are risks. But I'm no lawyer...