Brakes

I have a scenario in which my friend's brakes on his car are malfunctioning. It is not to the point where they don't work, but they are very weak. In my opinion, he will soon need to replace these to reduce the risk of danger in the future. From a personal perspective, people should really watch the parts of the car especially the brakes in order to be in the safest situation possible. People do not want to be driving with brakes that are not working correctly because that is just plain stupid. Therefore, I am going to warn my friend of the danger he could possibly have so that he can fix his brakes. Can anyone suggest a price range it will cost for a new set of brakes?