You didn't try putting the dll files in the plugins folder, while Winamp is running, and somehow, forget to even consider the possibility that Winamp may need to be restarted, did you?
If I recall correctly, I put libvorbis.dll, libmpg123-0.dll, and libg7221_decode.dll in the Winamp main directory. This was back when I used Winamp. Feel free to try this method as I believe it worked for me.
As punk7890-2 said, you need to put the other DLL files in Winamp's main folder, not the Plugins folder. The only DLL that goes in the Plugins folder is in_vgmstream.dll. That will probably fix your problem. Mouser X over and out.
Plus, if he's using any of the DLLs that I built, he'll need to be running at least Windows XP with Service Pack 2 or greater. Possibly Service Pack 3, I can't be too sure.