How extensive is the disability on her left side? Specifically, you say she can't use her "left hand." Can she use any part of her left arm? Maybe the forearm or the elbow?
What I'm envisioning as a work-around is an arrangement where her right hand still manipulates the crochet hook, forming the stitches. The yarn feeds through a "third hand" or "stitching bird," which is usually an embroidery or hand-sewing accessory.
This is simply one example of what I mean. An Internet search of "third hand" "sewing tool" will produce many different models, and there are yet more "third hand" tools for electronics, bicycle repair, and other tasks. If she's working with a physical therapist, the PT may have some good input on what kind of "third hand" tool would be most useful (if the therapist has a clue about crochet, that is).
Then, with her left elbow or forearm, she can hold the ball of yarn or the in-progress crocheted item. The exact mechanics will need to be worked out, and neck/upper back comfort attended to, but the "third hand" may help out.