If she is used to and likes XGA that would probably be the best, running an SXGA+ at anything other than native resoultion is not great.
I agree 100%, this is excellent advice. Resolution is such a personal choice. Each has it's advantages, and just because XGA is the oldest and lowest resolution does not mean it's not a great solution.
Most web pages are designed for XGA and display best at this resolution. If one spends a lot of time on the web I would highly suggest staying with XGA.
There is a "fad" in some cases of going to the highest possible resolution as it does produce images that are a bit less "grainy" and yet what good is that if you get eyestrain because of the very small fonts and images that are produced at the higher resolution.
Using myself as an example I have two T series ThinkPads that are configured identically with one exception. That is resolution which in one is XGA and the other is SXGA+. I prefer the XGA by a very wide margin when it comes to doing research on the web, lots of email etc. Then, conversely when I'm doing graphics work, modeling and rendering, 3D etc the higher resolution is preferable for those jobs.
Thus it's a matter of assesing honestly what you are going to be using the computer for and choosing the resolution carefully. Remembering that the higher the resolution, the smaller the fonts, icons etc. The argument that more can be seen with less scrolling by using a high res display is true, but at what cost, in terms of eye strain. Also its' important to note that I have perfect 20/20 vision. So it's not like I have poor eyesight. I make my living in front of a computer and have a very comprehensive amount of experience to draw on. And after owning a huge number of ThinkPads which is the only computer I use (no desktops) I find that the two best resolutions for me are those I have discussed and for those reasons.
If one is purchasing a ThinkPad for all around usage and at least 30 or 40% of the time will be on the internet, the most confortable and suitable resolution is 1024x768 = XGA. It will also save you some money, as the purchase price of these ThinkPads are less than the higher resolution models.
In the end you will be very happy you made that decision, as the higher resolution is often much more than one needs and is actually a detriment to enjoying a good experience with your new ThinkPad.