Duo's selection for me only gave "he slows the car" as an option. However I think the original "brakes" was more accurate (altho it could have been confused with smashing the car like you said!) because you can slow a car down using gears etc, whereas "freia" seems to specifically mean "to use the brakes". And im assuming you can "slow" a runner down using "Freia"? Any thoughts Paulo?
There's nothing wrong with "brakes the car" Do people teally say it? Hardly. He applies the brakes is what a speaker of English would most likely say. I brake is great. Abd stop the car is what the joe average would say. Remember though that Duolingo is a more "artificial" setting for language learning.
I am sorry, friends, I am a native speaker of Brazilian Portuguese, and "ele freia o carro" means literally "he applies the brakes to the car". Obviously, when one brakes it, the car will slow down, but it is not a good TRANSLATION of the given sentence. Moreover, Duolingo DID accept the expression "hit the brakes" in a previous question, to translate exactly the same grammatical structure (even though I did not find it quite precise). As a side comment, some colleagues have produced comments which suggest they might be confusing the English verbs "to break" and "to brake"...