Does the ACT determine how well you do in college?

So one of my buddies got accepted into Ohio State but he is concerned that he will have a difficulty getting through college because of his low ACT score. He did the test optional option and got a 22. He got in because he has a good HS GPA and does a lot of activities outside of school. I think he is under the impression that ACT determines your overall chance of success in college and thinks OSU might be too rigorous for him. Any advice for him?