Holy Cow! I would graciously accept the stash and anything that they offer. I would offer, as you have done, to help with the sorting, being sensitive to their recent loss. ("Your mother had great taste,,,this is beautiful fabric. ...I am sure that she wanted to make wonderful quilts for you and your family with this fabric. ...You are so fortunate to have ahd such a wonderful artist in your life.")
I would find a spot to store whatever I want to keep. Then, as many others have said, pass it on to other quilters or guilds---someone who needs or will appreciate that fabric will appear when you need them to. Do not let any of that quilting "stuff" go into the garbage dump.
I truly feel that it was meant to be.