Even if you throttle his connection to say 1/3rd of the bandwidth the router will run sluggish with all the connections. Set his max connections in his bt client to no more than 30 and the max upload/download speed to 75% of you upstream/downsteam bandwidth. Many people leave this setting untouched and this slows you connection down alot as if you are uploading at your full rate then dns requests etc etc run very slow so your web browsing crawls to a very slow pace. You can download and maintain quick browsing with the proper settings.
If your router has qos then you could prioritse ports 53, 80, 25, 110 etc etc to have garunteed bandwidth, if your router supports it.