All of the other Realtors who have answered this so far have good answers, but like Vicki said, it depends on the individual contract. For a Realtor to sue you, there must be a signed Buyer Agency Agreement stating certain terms and then the agent would have to prove that you violated the terms of the contract. We know so little about your personal situation. However, as Realtors, we strive to do our personal best to uphold our profession. This includes doing things in an ethical an legal fashion, although most of us would agree that a lawsuit in this case just doesn't sound right and most of us would never sue or have rights to sue a Buyer who didn't purchase a home. If a Realtor does file a suit against a client, the Broker and the firm's attorney would be the first to know and the first to find out whether or not there is "legal cause". Sounds like this Realtor and her client should have broken ties long ago.
I would answer differently if a purchase was made that went around the agent.