Hi Michael. It sounds like you are doing some great work with your students. I enjoyed reading your post. I hope that your student who did sell her work was over the moon. What a great accomplishment.
I used to work with adults who had cognitive issues due to dementia or head injuries. It can be a challenge to find visual art activities that are safe and appropriate for an adult and that produce satisfying results. Have you considered clay? I know that there are easy recipes (sorry I don't have one to share, at hand) for making basic modeling compound out of edible ingredients. If she chose to taste it, it wouldn't matter and a little saliva won't compromise the working characteristcs of the compound. Once you have the basic clay, it can be dyed with food colouring. What about adding scents with baking extracts? What about adding other media like metal jingle bells (large enough not to be a choking hazzard), or crinkly cellophane? You could make a lovely collage-style wreath by sticking clay and other things to a pre-made armature.
If you are working on a piece for a show, I'd suggest offering a limited number of colour choices, in the theme of your show (i.e. red, yellow, green, white for Christmas). That gives your student some choices and helps ready her work for the show.
Maybe flour and water papier mache (sp?) would be an option? It is squishy and fun!
You can add textures with glue and salt.
Crumpled cellophane also makes a nice, loud paintbrush.
Does the end product need to be a functional "thing" for a show or is an abstract collage o.k.? Is there a music therapist that you could consult?
Good luck. I am so glad that your student has a caring teacher who's not giving up on her.