If one of my tires got slashed, it is alright to just change the one tire? This actually happened to my sister and the car has only 13k on the original tires. I know you can't on a car like a Subaru with the AWD, but does it matter on a FWD car?

 

Probably no big deal to do that. Unless she's some type of crazy driver or something.

 

How worn are the tires now? If the present tires have good tread, then you will be okay. If not, then you may want to get two tires and put the new ones on the front. Even still, like Jeff92SE said, unless she is a crazy driver or super-sensitive to how her car feels, you probably can get away with just one.