Answer:
First of all, when a buyer or a seller agree to work with a broker, they must first sign a contract that binds them. If the seller's broker failed to perform his/her part, it is something completely irrelevant to the buyer's broker. They will probably engage in a legal dispute over the brokerage fees involved, but that is someone else's problem.
The buyer's broker actually did a good job according to the seller since he/she bought the house at $100,000 below the market price. The buyer should be happy with his/her broker and should pay with a smile on the face.