Risk of autosomal dominant disorder
Your classmate is forgetting that the child will have to inherit one allele (A or a) from each parent. Suppose the dad is affected---so his genotype is Aa. He has a 50% chance of passing the A on to his kid. That means the kid has a 50% chance of being affected.

If your classmate was right, then it would be possible for both the kid's alleles to come from just one parent. You have to pick one allele from each parent.