If the cast is over his/her elbow, I can’t imagine how he/she can do curls. Otherwise if the doctor allows her to move the arm, she could do modified curls with less weight than before. Don’t forget the cast has its own weight. Also remember that if she is newly injured, she might have to abstain form elevating her heart rate for a while because that can cause stitches to pull.
On the assumption that the client has been cleared to do any weight lifting:
I would use a cuff to attach around the wrist and then do exercises on a cable machine (or tubing/dynabands).
I also use this modification with a client whose hands are very arthritic, and she cannot hold weights very easily. I do not know what you have available; at my studio, I use this technique also on the Bowflex and on the Total Gym Power Tower.