The Spanish contestant was told of Williams's backing when she met his friend Jonathan Wilkes.
She also had a childhood dream fulfilled when she met Take That during this week's masterclass.
A show spokeswoman said: "Ruth admitted to the guys that when she was nine she wrote to them asking them to sing at her birthday party but never got a response, so as a belated birthday gift they sang to her in Spanish."
Take That will perform on tonight's show, along with former finalists Rhydian Roberts and Same Difference.
The X Factor airs at 7.25pm and 9.55pm tonight on ITV1.