What error message does he get?
If Dx9 is not installed he should get a message as such. If he doesn't have at least the august 2008 update of Dx9 it will crash when loading the xaudio2 plugin, but it should give an error message like "failed to load 2.csx". The visual c++ 2005/2008 redistributables are usually not needed as most of the plugins don't use them, but if a plugin does use them and the files aren't installed he should get a failed to load plugin message.
You can use dxwebsetup.exe which is included in construct's install directory to get the latest Dx9 update.
You can download the c++ redists here:
But these are not normally needed as I belive most of the plugins don't need them.
You can also manually check a plugin to see what dlls it need with "dependency walker":