I have both the Vizio 50" plasma and 32" LCD..
The main thing that bugs me about my plasma is the image retention issue.. If you leave something on the screen for too long, such as a paused DVD, the screen will retain that image until you've watch enough other content to wipe the image away.. And eventually the pixels in the screen will wear out and the picture will become dull.. LCDs don't have this issue..
Also, I personally think plasmas look worse on standard definition channels than LCDs do..
LCDs tend to be a little more pricey than plasmas as you go up in size..
The best thing to do is to have her look at the picture of each type of TV and see which one she likes the most.. And she needs to know that what they're showing in the store is more than likely HD content, so it's going to look great (or at least much better than what she's used to at home)..