PLEASE DO NOT use Black Salve on your Cat! It is VERY painful and will eat in as far as the Cancer is, and if it's too deep your Cat may die. The pain alone may do this. I have used Black Salve on myself and I know how it works. Good stuff if used in certain places on your own body, but there are better ways for your Cat. Please don't do it!
Get some Lugol'sIodine and start putting a couple drops in her water.
I hope Luella or DQ will see this, they have great ideas regarding Cancer. They might tell you to start with Colliodal Silver.
Please NO Black Salve on your Cat! Save it for yourself!
*EDIT* Since the Tumor is as large as a golfball, that is even more reason NOT to use Black Salve because it will eat in far deeper than that Tumor and be agonizingly painful*