I am no expert, but you do need to find out a bit about them before you buy one, especially if it is for someone who doesn't have much computer knowledge.
If you buy a frame with takes the same type of memory card as your camera, then you can take it out fo the camera and into the frame, but then you can only use one of the them at a time. BTW, there are different types of memory card, you can find out what type is in your camera from the manual.
What do you or your mother currently do with the photos from your digital camera ? Presumably you save them to a computer.
What you really want to do is be able to browse the photos stored on your computer and save the good / memorable ones to the memory card of the frame. Doing this via USB is probably easiest, so you will need to buy a device to save to the memory card via USB.
I probably haven;t answered your question, but I tried to point out the things you need to work out before you buy a digital photo frame.
Maybe you should look at
www.howstuffworks.com and read the section on digital photo frames. It might help