Sympathetic as I am for your friend and his large phone bill I cannot believe that he was naive enough to think that he could use his phone on the internet for unlimited downloading for a one of payment of £5. If that was so why wouldn't everybody be doing it to access the internet and sending emails etc.
The key to this seems to be the warning "When connecting a message on my PC explained you will be charged for internet...... etc." I suspect that either the warning message wasn't clear enough or your friend didn't read it carefully enough.
I don't think that he has a leg to stand on and unfortunately will have to pay up. Perhaps he can come to an arrangement with 3 to pay in installments.