I agree with the RAM suggestion.
Generally Apples don't really slow down like windows machines through their lifetime. You can't really do defrags because the OS does disk management better so it's not really a problem. It can be done but it is dubious as to the result.
However you can do a permissions repair which may help (I'm told). It's never done anything for me but maybe before bed one night it might be worth a try.
Open diskutility. Use spotlight (Apple-space) and type in "disk utility". It should come up as an app. Hit that. Select mac hard disk and then click repair permissions or verify permissions. It might take a while. :)
EDIT: Also is she starting it and re-starting it every time? If she doesn't remind her to re-boot it every few months. If she does re-start it then tell her not too! Macs have **really** good sleep modes. Just leave the computer when she is done with it and it should pop to sleep in a few minutes (unless someone has tinkered with it). You can also tell it to do it or close the lid. When she wants to use it just tap a key or open the lid. It will be right for action in <5 secs!

