The Ninety-Ninth Commencement took place in 1990. Ndesandjo is listed on page 84 of the 1990 commencement brochure. The top of page 85 is included for reference.
Thanks to one of our intrepid researchers, we now know for certain when Mark Okoth Ndesandjo, the half-brother (?) of Barack Obama, was awarded a Master of Science in Physics from Stanford University: 1990.