If they don't want people hosing their internet connection then they will need to shell out a little bit of cash for something a bit more advanced.
I have set up a few local places (cafe's mainly) with WiFi Hotspots and each time used their existing router (with the wireless turned off) connected to a
PC Engines alix2d2 board loaded with 2x
Winstron CM9 802.11a/b/g wireless cards running
pfSense. This provides 2 SSID's, one for their employees to use and one for the punters, on the punters network you turn on the
captive portal so they have to provide login details (managed on the pfSense box or external authentication provider) before they get access to the internet - this lets you monitor the traffic they use. You can also limit the outbound ports the punters network can use so they can't download on BitTorrent/Newsgroups, etc.
The whole hardware + software cost is ~£160 so it's not too bad, about the price of a DrayTek 2820n and can do a whole lot more.
The pfSense wiki has loads of tutorials on how to set up every feature on the firewall, if you get stuck the forums are really helpful. Feel free to drop me an email (in trust) if you want some more information
Also, You couldn't just plug a BT Business Hub in to their existing router as it's WAN interface is fixed to the ADSL/RJ11 port, you could however completely replace the existing non-wireless BT Hub with the wireless BT Business Hub, providing wireless to the punters on the Guest wireless connection however you don't have the monitoring, authentication and firewall that the pfSense box has.