There are lots of 4 cylinder cars that might fit the bill.
You should be able to get a reasonable guesstimate of FE for anything you/she looks at by going to
Fuel Economy - User Estimates
Example:Ford Escort ( be sure to click the Show All links eg Show Alll for 2001 )
Look for something your mom likes.
Mpg won't matter if she doesn't like the car!
How mileage-conscious is you mom?
And how many miles does she drive a year?
I suggest you make some tables based on that number of miles and gas at
various prices. eg
5,000 miles @ 25 mpg & $4 per gallon gas = $800 of gas.
5,000 miles @ 30 mpg & $4 per gallon gas = $667 of gas.
5,000 miles @ 35 mpg & $4 per gallon gas = $571 of gas.
5,000 miles @ 50 mpg & $4 per gallon gas = $400 of gas.
5,000 miles @ 25 mpg & $5 per gallon gas = $1,000 of gas.
5,000 miles @ 30 mpg & $5 per gallon gas = $833 of gas.
5,000 miles @ 35 mpg & $5 per gallon gas = $714 of gas.
5,000 miles @ 50 mpg & $5 per gallon gas = $500 of gas.
(5 years)
25,000 miles @ 25 mpg & $4 per gallon gas = $4,000 of gas.
25,000 miles @ 30 mpg & $4 per gallon gas = $3,333 of gas.
25,000 miles @ 35 mpg & $4 per gallon gas = $2,857 of gas.
25,000 miles @ 50 mpg & $4 per gallon gas = $2,000 of gas.