I would ask her what her moms favorite colors were and see if you can add them in somehow. Or maybe see if you can find some fabric with items that were special to her mom. I know when I made my dad a quilt with my moms shirts, I added a ladybug to the back as she always thought the meant good luck. As someone who has lost her mom I think your friend will cherish the quilt no matter if it has one block with her moms things or ten. That one block will still feel like her mom is holding her when she is wrapped up in it.
Also maybe with the 12" square you could divide that up for both quilts and make the label out of that.