Recover data after reinstalling OS X?
The Apple installer does not erase the disk unless you tell it to - that involves opening Disk Utility. So your files are fairly likely to still be there in the Users folder, just under your old user name.

However, you said "She used the installation CD" - you have old hardware running OS X.6 (Snow Leopard) or earlier? those versions DID have an "erase and install" option.

Of course, the easiest way is to simply recover the files from yesterday's backup that you of course have on an external drive.