Phils right, DVD requires a MPEG-2 decoder which due to needing a licence generally requires you to have 'obtained' a piece of commercial DVD software.
Now if it was working before the chances are her computer or a component of the computer came with a DVD player, most of the main PC manufacturers ships any computer containg a dvd drive with something like PowerDVD or WinDVD so she just needs to install that. If she isn't sure check the discs that various discs that came with the computer/components
If she doesnt have the software then you could try out http://www.free-codecs.com/download/...EG_Filters.htm
havent tried it myself but it does claim to decode dvd's