Your dog needs to see that you are protecting her. Cats are fairly easy to pressure. Any intense action from you towards the cat should suffice. This could be an assertive noise, a clap of the hands towards the cat (not the dog), stomping quickly towards the cat, etc. They are pretty sensitive to pressures. Use a word at the same time like "leave it" in a firm tone - usually a loud "pissst" sound does well. Explain to your roomie that you love the cat but you need to help your dog see that you are willing to defend her. Plus the cat needs to learn not to pick on dogs.
Love Them & Lead Them,
~Elizabeth & Doug
Dog Training the Way Nature Intended