Your mother must have had doubts for a long time.
Otherwise, why did she keep asking Jehovah to show her if it's not the truth?
You might ask her, "If Jehovah was trying to show you it's not the truth, what sort of evidence would you accept?"
Perhaps she'll see that the only way for her to know whether or not it's the truth is to examine it carefully and be willing to go wherever the evidence leads her.
Would she be open to listening to what Ray Franz had to say in Crisis of Conscience?
He was on the Governing Body, after all.
What better evidence could Jehovah give her?