"...his plan clearly stated he had unlimited web browsing from his phone, that's totally distinct from using it as a peripheral device connected to a PC..."

It may seem so to you, maybe. To the general user, "from his phone" includes "via his phone". And it's no surprise that the telco just merrily let him run up such a huge bill. If there's a limit on how much traffic can be passed "inclusive" there should be a warning as soon as that limit is exceeded, and probably one saying it's being approached.