Selling a property whilst a deal is pending

My mother-in-law (78 years old), signed the contract to purchase a property in Bougainvilla Retirement Village on 17 March 2015, the deposit was paid on the same day, and the deadline for guarantees was 30 March 2015. On 26 March the agent phoned to say, sorry they sold the property to someone else. We are also still waiting to get the deposit back. We have never been so shocked, no regard for a signed contract, the fact that a transaction is in process or for basic business principles & client service! They also mainly deal with the elderly, which makes this worse - maybe it is an easy target for them!