I like a BB gun too, but she will get smart about it. Once they hear the BBs rattle in the gun they take off. But it will work.
I like a shock collar, but then again you would be spending money for a dog that is not yours.
One method I use for a dog/pup that likes to work before I give the word..
I will put them on a long line and when they go to take off for something without a command I will call them back but if they ignore me I will yank them back with line. The long line makes them think they got far enough away to ignore you without the risk of being reprimanded. Then they learn to listen no matter how far away they are.
But then again you are training your cousins dog which does you no good unless she follows through as well.