I would suggest Shiva oil paint sticks. There are all kinds of sites to get information about the Shiva oil sticks being used on cloth. There might be a learning curve on using them with a brush, but I can't see why it couldn't be done. This way she could do what she wants without using regular canvas (it won't combine with quilting fabrics). She needs to make sure all the sizing has been washed out of the cotton fabric first so the paint can penetrate the fabric.