@chscag:
Actually, iWork '08 can't open files created with iWork '09.
@Chippy22:
for future reference:
Your sister should have gone to File > Save As > Save a copy as... selecting either "Word document" (.doc) or "Pages 08" from the pull down menu.
On your end, a workaround would be – as bobtomay suggested – downloading the trial version of iWork '09; after the 30-day trial period it turns into a simple viewer (which means you can still open documents but you can't save).
Another quick workaround would be to change the extension of the file from ".pages" to ".zip" [don't compress the file, just change the extension]. If the extension is hidden, the easiest way to do that is from the file's Get Info window in the Name & Extension field. Make the change, hit Return – or simply close the Get Info window – and accept the change to ".zip" when asked). Then unzip the file. In the unzipped folder you'll find a QuickLook folder and inside that a pdf of the document you were sent. That can also come in handy when someone is on a pc and can't have iWork.