Give the guy a break, it is entirely possible that all of his downloads are legit, you are relying way to much on speculation.
374GB is really not that much, it's only 1.19Mbps sustained.
Concerning the "unlimited" usage; This is basically a marketing ploy that 99% of the time works out for both the consumer and the provider. Some users will naturally use more and some will use less, in theory it should all balance out in the end. The advantage to this is the provider/consumer needs not to worry about hard limits, but the disadvantage to all of this is that the upper 'limit' becomes fuzzy and disagreements will arise.
It's kind of like phones, everyone basically pays a set price for their line, however not everyone can be using it at once. Imagine the confusion of the pricing structure if this type of marketing/pricing was not allowed?