I just saw a friend's collection of coins with dates that are prime numbers. I thought it was a clever idea. Here's a site that shows the primes between 1901 and 2000 plus links for other ranges. https://www.easycalculation.com/prime-number-chart-1901-2000.html
If I have that right, then your friend only collects coins from
1901, 1907, 1913, 1931, 1933, 1949, 1951, 1973, 1979, 1987, 1993, 1997, 1999 and other prime number years?