If your friend has Photoshop, a good way to go about it would be to first have the photo in a layer separate from the background:
- Then she would have to click on that layer in the 'Layers' panel with the ctrl key pressed. - This should put the entire layer under selection. Right-clicking on the photo itself should now enable the 'Transform selection' option from a drop-down menu. - If you resize its width and height down to 50%, you will get a selection that will fit in exactly each quarter of the photo. It can be moved around with the marquee tool selected; it will automatically snap to corners. - With that, you can copy each corner of the photo and paste them into separate layers - a photo split into 4 even parts.
My apologies if these instructions are excessively detailed. I just assume I shouldn't skip anything if I don't know how familiar the person is with Photoshop. Hope this helps.
If she has photoshop she ought to be able to turn on the rulers and adjust the size there. I'm awful at math but 4 8x10s would come out to 16 x 20 if I'm not wrong. She should resize the picture to that, then use the rulers to make guidelines (usually just clicking in the ruler and dragging onto the image will make a blue line that won't show when printed). Then crop each section into an 8x10, save it separately, 'undo' so she's back at the original, then do it again with the other three. This is probably not the easiest way to do it, but it's how I'd muddle through XD