Hey there, first of all, it would be important what app has used that traffic. Or does your friend watch a lot of YouTube videos? WiFi will use the WiFi traffic. If there is a flat rate, everything will be fine.
You can turn off data usage. Go to Settings -> Wireless and networks -> Mobile Networks. There you can switch off the data traffic. You might find APNdroid
interesting as well. This app can help you to turn it off more easily.