If she refuses to change the number, put an automated answering service on the number which instructs people about the fraud or if they really want to talk to the little old lady to press 1. That way the angry people will get told its all being spoofed and wont make the threats. You can do this fairly easily with Asterisk too. Cost wise, Asterisk is free, the pc is the main cost if there isn't an existing one and you can get the required Asterisk hardware (pstn -> pci card) from Digium.
If the govt won't help then this is the best solution without changing the number.