I was wondering if there is a way to recover photos and things alike from a MacBook after reinstalling the OS X.
Like when you reinstall Windows there is the "Windows.old" folder and you can recover some of the data from there.
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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.
After searching for some hours I found a program which can recover files. http://www.cgsecurity.org/wiki/PhotoRec You select which hard-drive will try to recover data, the format in which files are saved in that disk,... Also you can select which types of files you want to get. For example .jpg. The program will look for files in the hard-drive which have the header for the .jpg and will save them in a folder. The only bad thing is that it recovers also program icons and everything so you will have +30k photos which you won't need.