Check with your friend what size pillows she wants these cases for, too.
If your friend is 'cheap' - have her buy the fabric -
If you choose to make 'plain' pillowcases - for a standard pillow (20 x 26) - with a four inch hem
You would need a width of 41-42 inches - depending on how loose you like the pillow case and/or how wide you make your seams - and a length of at least 34 inches.
The standard queen size pillow is 20 x 30 - and that would take a length of at least 38 inches.
You can skimp and make narrower hems and that would take less fabric.
You could make only one side of the pillowcase with the batman fabric - but that would mean a seam on each side, instead of a seam just on one side.