As I understand it he is transfering the images (probably ADF, not sure) from the PC to the Amiga 500 via Amiga Explorer. Apparently the ones that are recognised by AmigaDOS work, but the games that aren't don't. I am assuming he is transfering ADFs and using ADF2DISK to unpack, that's what I recomended.
Personally I'm not terribly familiar with Amiga Explorer, I used to use TWIN extensively and not I use ethernet or Catweasel.