Posted 16 October 2012 - 07:10 AM
The first thing I would check is to verify that your wifi connection is secured. If it is open, someone can use your connection.
If it is secured, I know there is some malware in existence that uses your bandwidth for part of a bot-net. You may want to ask in the Malware help section for this.
Is it possible that your friend is actually using this bandwidth? For instance, if you were to watch ~8 HD movies you would be getting close to that level of bandwidth consumption.