Firstly, apologies if this is not the correct place to post this...
A mate of mine works in a bar and they've recently had (BT) broadband installed.
The router is located near the phone line and provides wifi for most of the bar area. The office of the business is behind a couple of thick walls and the wifi signal doesn't reach, nor does it reach the back area of the bar. They originally tried getting a usb dongle to provide wifi to the office computer. With a 5m cable it gets a very weak signal at best and none at worst. They then got hold of one of these usb booster cables to attempt to extend the usb cable to 10m (that didn't work).
I have suggested to them to get a booster/repeater that would improve the signal to the back area and allow them to run a cable through to a network card in the office computer.
I believe that the router provided by BT is G rather than N.
If they need a good strong signal in the office area, and there are walls involved then your best bet might be Ethernet cable. If the office is within 100m of the router a cable could deliver a solid connection.