How many miles?
When I bought my 00' with the Series II 3800 NA it only got about 18mpg. I changed the fuel filter and now I am getting about 20-22.
Although I only got 19 on this trip after running it a little hard 
. (these cars are fun to drive)
I would change the fuel filter as the easiest option. You will not get the nice 22+ mpg since you are primarily in the city.
Tell your mother to not be afraid of the 3800. The engine is very reliable and MANY people swear by it.
Let us know how things turn out.