I was assuming he doesn't have unlimited data. (My plan is unlimited, but I'm very careful to research other people's success doing this with my carrier, and I don't abuse it.)
With a limited data plan, the price should be cheaper than getting a dedicated hotspot with its own plan. I'm also really doubtful the carriers' restrictions on tethering would hold up in court if a plan has a data cap. You paid for (say) 5 GB of data a month. What does it matter to AT&T if you use that 5 GB on your phone or on your laptop tethered to your phone? 5 GB is 5 GB - it represents the exact same use of AT&T's network regardless of if the bits end up on your phone or your laptop. Their reasoning trying to market it as a different add-on service is idiotic. It's like buying a gallon of milk at a supermarket, and at checkout they tell you that you are only allowed to drink this milk from a cup. If you want to use it in a bowl of cereal, you have to pay extra.