1998 Oldsmobile Aurora poor millage after tune up??

recently my mother had a tune up done to her 98 oldsmobile, new plugs, new wires,oil change,and new O2 sensor and even had new tires put on. But now after that her gas millage has dropped from about 28mpg to about 18mpg. Is there anything that could be causing the poor gas millage. She is still using the same type of gas and isn’t driving more agressively. Kind of at a loss to why it would be getting such poor millage after everything that was done to improve the running condition. And she had the gas lines and tank checked for leaks and there were none. I have been trying to think of another reason but im at a complete loss as to what it might be.

thanks