50% if the second child is a boy, 0% if the second child is a girl. Overall, 25% chance.
Work Step by Step
Here, make a Punnett square for a monohybrid cross. Since the trait is sex-linked and Tim lacks it, he must be D- (one allele for normal dystrophin, one non-allele on his Y chromosome), and Rhoda must be Dd. Her d allele passed to the first child born. Girls will always get Tim's D allele and not have the disease while boys have a 50% chance of getting the d allele and a 50% chance of getting the D allele.