So far, he has succeeded in getting RGUI rendering onto the screen, the PS2 joypad is working, and a core has been compiled (Picodrive). From here on out, the challenge becomes a bit harder - he will need to figure out sound, get the libretro core actually running, get audio/video performing well, etc.
You can see here this current WIP video of how RetroArch on PS2 currently looks like. RetroArch is a project that prides itself on being available everywhere. While most projects look forward and abandon the past, we manage to walk and chew gum at the same time, and we don't sacrifice backwards compatibility on the sacrifical altar of 'progress' either.
This port further reaffirms how important it is that RetroArch sticks to tried and true coding practices so that it is infinitely downwards portable.