I don't think the OP is interested in the STM rules...
Chris, VM do not have a policy of restrictions on the total amount downloaded, though I will state that as your father is using a residential service for business they could in essence restrict the account purely on that if they ever found out.. (and yes there are ways especially when downloading from a business address all the time)
He could always get a business account (£49 for 20Mb, I believe it is STM'd unlike the 10Mb but with a lot higher limits and no limits during the day) but for that you also get a phoneline with unlimited calls at any time of the day