thor220 :
bill001g :
The simple but still pretty stupid monitor is the resource monitor built into windows. It will give you the rates in bits/sec of which you just need to multiply by 450 to get rate/hour in bytes.
Games use very little a couple hundred megbytes/hour. Netlix watching HD can easily use over 2gbyte/hour. Of course I am sure you know there are games that are over 20gb downloads so those can wipe your cap in a couple hours if you can get good transfer rates.
That will only get the data usage for his PC. It does not include any other device, of which there are many with 4 people in the house.
Of course it won't you have to run it on each device. I just did not want to go into the complexity of recommending loading third party firmware into the router so the router could collect it.
Your assumption that his ISP has some magic web page that can give him the information is just as invalid.