George McLaughlin, of McLaughlin Motor Factors in Buncrana, said he was disgusted by the attack.
Mr McLaughlin said he has his own personal reasons for offering the cash reward
“My own mother lives alone and I would not like to think that something like this could happen to her.
“I will quite happily give this reward if someone is caught. Perhaps some of their friends or people who know them will be tempted by the money,” he said.
The next-door neighbour of the 96-year-old woman, robbed and punched by thugs, believed she was dying when he heard her screams of pain.
Robbie Callaghan, 52, rushed to the aid of frail Ms Lilly just seconds after she was attacked at her Co Donegal home.
He and wife Rosemary were entertaining friends at home when Ms Lilly’s alarm was activated on Saturday morning last.
Robbie, who is linked to Ms Lilly by an Eircom alarm, rushed to the scene and found her terrified with fear.
“The first thing I noticed was the kitchen window was open and I knew that wasn’t right,” he said.
“All I heard then was this screaming and I called out her name and she recognised my voice.
“She then started screaming that she was robbed and attacked. When she opened the door she was holding her side and it looked like she had been hit on the face,” he said.
Robbie, who has lived next door to Greta for almost 20 years at Aghilly, Buncrana, said he has never heard or seen such terror.
“What sort of person does something like this? It’s bad enough she was robbed but to hit a woman who is almost 100-years-old is just disgusting,” said Robbie.
Ms Lilly was still being treated at Letterkenny General Hospital yesterday but in spite of the ordeal, her injuries are not understood to be serious.
The two men who carried out the attack wore dark clothing, balaclavas and spoke with local accents.
They are understood to have escaped with a substantial amount of cash which Ms Lilly was saving for new windows at her home.
Her husband Ted died around 10 years ago and Ms Lilly has lived alone since.
Meanwhile, gardaí say they are making some progress in their investigation. Inspector David Murphy of Buncrana Garda Station said: “We have made some progress but we have not made any arrests at this stage.
“We have received a lot of information which we are following up and acting upon as part of our overall enquiry,” he said.
One of those leads is a caller to Ms Lilly’s house last Friday who asked to buy an old car in the driveway.
It is understood the man insisted on buying the car for scrap and gained access to Ms Lilly’s house at one stage but then left.