Ritchie

Those three pieces of kit will do exactly what your friend wants.

Why modem/router? Because the original modem I would guess has only got a USB port and thus won't connect to the router. On top of that, when setting up his network it is recommended that you hardwire everything together first using ethernet cables and then, when it's all working, switch to wireless.

The setup as envisaged would allow file sharing anyway. With both the laptop and the desktop connected to the router you can install file sharing between the two PCs. Might have to play around with firewalls and stuff but it will work.