To determine the top 5% of the population, she needs to do a 90% confidence interval for the population. The region in the interval gives 90% of the population. The region above the interval gives the top 5% while the region below the interval gives the bottom 5% of the population.
90% C.I. = mean + or - 1.645 x (standard deviation / sqrt(sample size))
Therefore, the top 5% are the region above mean + 1.645 x (standard deviation / sqrt(sample size))