If she is not going to upgrade often, I would invest in a newer model. As new iPads and phones come out, new apps stop working with older software. While you can upgrade your software for free, eventually you get to a a point where your system just isn't built to run the latest software and you will lose access to some apps or have to upgrade your hardware.
The latest iPhone is the iPhone 5S with iOS7 (also for the iPad) and iOS 7 is not compatible with the iPhone 3GS or anything earlier. The iPhone 3GS came out in 2009 and was replaced by the iPhone 4 in 2010, so that gives you an idea of the timeline.
If you aren't going to upgrade often, I think it's worth it to get the newest hardware to give yourself more time to keep the software current.