Hi cyberphobic, filing on the OS X operating system is similiar to Windows, so dragging and dropping the documents, videos and pictures into folders in OS X should be self explanatory. There are even folders specifically made for videos, documents and pictures, so just plug your hard drive into the USB port (or take advantage of Mac's Firewire port) and get dragging and dropping.
Applications however will need to be downloaded again, as the operating system is different to Windows, so make note which applications she had on her Windows PC and download the OS X versions.
Transferring a iTunes library from a Windows to OS X shouldn't be different then in Windows to Windows. Here's a guide: