How is he not paying anything? If the bill isn't paid, do they not charge interest on his account and eventually send collection agencies to him? Of course he's paying!
What did D (friend) do to cause B to be obligated to pay money? (Even principal)
He swiped a card. The card is an agreement btwn B an A that says that A will give C money.
When A does give C money, this causes a hischayvus between B and A, this hischayvus would not be a chov rather a hischayvus midin eved.
The $48k question is:
When D causes the hischayvus of B to A, under what din does D now owe B money?