He told me that he was speeding on the highway.
Is the highway faster than the other route?
I bet it could be, I've never checked it out.
If I were him, I would slow down. Breaking the laws of the road can get you into serious trouble.
If he is doing fine then forget it. Everyone needs to deal with their own problems.
If you are not concerned, I am surprised that you brought the problem up to me.     If you are concerned, you should tell him in a constructive way.