A friend told me his friends drive 90mph on the highway and that they rolled down their windows and discussed where they were going while driving side-by-side at that speed. He never actually said whether he was in the car with them or not but at the time I assumed he was. All I said in response to this was "Uhy-yuy-yay" or something to that effect. I thought of saying more to dissuade him from riding with them such as: "That's really dangerous" but I didn't because it might have been uncool or something.
I did not specifically think of it as a sin at the time but did I participate in my friend's objectively grave sin (putting himself in serious danger by riding in the car with his friends [is this objectively grave?]) by not saying more? Is this a mortal sin?