I am sorry she paid the Broker Fee which is generally not due until a lease is signed (not earned by showing an apartment).
First stop is to call the Realtor's Broker and file a complaint. If this is to no avail, let the Broker know that you will pursue the issue not only with the local Real Estate Board of which they are members, but will file a complaint with the New York Department of State.
Good luck with it.
On the other side of it all, crimes can and do happen everywhere, unfortunately. And although I am sure it made your mom feel uncomfortable, she may want to reconsider finalizing the lease.