If 256MB is the maximum memory that can be installed then the chipset may not be able to address more than half the memory on the newer high density memory modules, or even recognize them at all. I have an Ali Aladdin V (M1541) chipset motherboard that only recognizes 64MB of any 128MB high density module I install. A freinds Dell Dimension L500r with an Intel 810e chipset won't boot with the same modules - just returns an error. 256MB is the maximum addressable memory for both boards so they seem to be about the same vintage as your friends.
The specs for a motherboard will usually list the chipset. Why don't you post a link to the model of motherboard at the manufacturers website. Or the make and model of the PC if it's an OEM build. Tech support at Kingston or Crucial may be able to advise you once you know the chipset.