There is an easy way, and there is a hard way. The easy way requires you to get access to her computer and download
Everest Home Edition. After installing the application open it up and find out if you're currently using SDRAM, DDR, DDR2, or something else. You will have to match the memory in order for it to work together.
Now for the hard way. You will need to find a way to open the computer to take a peek inside. Pop out the memory stick(s) by opening the pins holding the memory shown in
this picture. Afterward look at the bottom of the memory stick(s). You're looking for the noticeable difference between DDR and SDR as depicted
here.
Once again, you will need to match the memory types to ensure that they will work together. Also, please don't insert an odd (3, 5, 7, etc...) number of memory sticks (1 stick being the exception), some motherboards can't handle it.
Let us know what you find out. All the best in helping out ***********-in-Law!