I have a friend who is addicted to drinking brake fluid...

I have a friend who is addicted to drinking brake fluid.
He says he could stop at any time

This is a weird joke. The phrase "I am not addicted, I could stop at any time" is often in american culture uttered by people talking about addictions. That being said, brake fluid is a fluid that helps cars brakes to operate (which makes the car stop), so if you drank brake fluid, it could (in some absurd illogical way) be expected to make you stop.

I have to admit that I like this joke.