Welcome to the forum, PiggyInClover.
I am afraid that's the only useful thing I can say. You seem to have done your best - rather more than I would have done, I think. This old cynic would have given up six lessons ago, especially if I were doing it free, as a favour to a friend. If Student B doesn't have the common courtesy to show some form of civilised behaviour in return for what she is receiving, then (expletive deleted) her.