Apart from the addiction factor which is far from healthy (although having succumbed to the odd "click-frenzy-fest" myself now and again, totally understand the allure), usage will be highly variable. Most YouTube videos are at 360p or even 240p which, even for a full day, might only clock up 1GB (depending on how long a "day" we are talking about, of course...). However, once one starts heading into 480p, 720p and full HD, all bets are off, and one can really start chewing thru the monthly GB allowance in short order. If you don't feel this guy's addiction is unhealthy, just ask for $5 a day to cover his usage (which will include sundries like the cost of the electricity to run the router / computer) and, provided you don't mind him slowing down your connection (when it comes time for you to share it with his 'tubing), that might help to either put the brakes on or help you pay for it.

Telstra are pretty expensive for their GB packs, but not massively so. I've swapped to NOW (not sure how available they are outside HB), and I get 75GB a month incl phone for $73 - and if I've not used any of the 75GB it rolls over to the next month. Even after very heavy 'tubing (in HD, incl several Nat Geo 1.5 hour docos) over the last 3 weeks, I've still got 94GB available.
