Windows Paint can do this (resizing) easily, and I think there is at least one other built-in Windows photo app, but can't say what it is or does. You can open a jpeg in Paint by right-clicking it, going to Open With and then choosing Paint, or by going to the Start/Run function and typing in "pbrush".
But, she also can check out the Canon software! If she got the disks with the camera, then she can install several apps. Three are of particular interest: ZoomBrowser and Digital Photo Professional (DPP) both have editing capabilities for jpegs, and DPP can process Raw files if she decides to go that route. Some of the DPP features are more advanced than ZoomBrowser's, so it is definitely worth checking out.
What I don't see in ZoomBrowser is a resize utility. But DPP has resizing built into its Convert and Save function as well as its Batch processing function. From Zoombrowser you can open an image in DPP, or of course you can directly open images in DPP using the Browser.
The Canon EOS Utilities app can also be useful. Many folks use it to attach the camera to the computer and directly download their shots (although many of us prefer to use card readers). I believe ZoomBrowser has this ability as well, but with the Utilities you can have your images open in DPP automatically so you can do the resizing, but also you can adjust the images for things like Brightness, Contrast, color corrections, Sharpening and Noise reduction, things which she will eventually want to be able to do!
Plus, EOS Utilities has a tethered shooting utility that has the nice feature of utilizing Live View, very nifty.
If she has the disks, installing these three programs can get her started, but I would advise her to use the second disk and to load the manuals, and get started reading them in her spare time!
If she doesn't have the disks, there are workarounds. Find out an let us know and we can help.
Once she installs the software, it would be best for her to update the three apps and the manuals by going to the Canon site and navigating to the software for her camera and the manuals for her apps.
And then, an app that can really help her to get seriously started is Photoshop Elements! Not only does it have the Resize, and also basic editing functions, but also some very powerful and useful features that eventually she'll want to learn.
Elements is a great buy at less than $100!