The reason is that the upgrade to Maverick was only very recent, but it is now clear that she has lost the use of such things as Photoshop, the Adobe CS suite, her easy to use Iphoto, some of her mailing programmes, past data relating to web sites, etc. And, of course, her substantial WD back up will no longer function until that is upgraded. In fact this upgrade has been something of a nightmare. for her and has resulted in a near nervous breakdown. I, myself am still running on Snow Leopard and have no desire to move ever upwards. My feelings are that if it isn't broke, don't fix it. Apple seem to be pushing hard to drive their customers into using Cloud applications and ditching many of the simpler things in life. Not everyone wants to join the fast set. I have recently come across someone who has many old files on Pagemaker 5 that can no longer be accessed.
Since this upgrade has only been recent there are very few files that would be lost by a backward move.
Thank you and everyone else for your input, and it seems that going backwards is not impossible, it just has to be done with care.