What are the risks for a cat that eats dog food?

One of the two cats I have has always been extremely picky. She had only eaten one brand of cat food for 8 years. Now all of a sudden, shes starting to eat the dog food. ( Im assuming because shes sick of the same-old-same-old. ) What are the risks shes running by eating it? She is a big cat - well over weight - but she always has been. What can I do to stop her from eating it, because like I said, shes really picky about what cat food she eats.