We don't see taking the snake as theft. But authorities won't see it that way.
Whether or not it is right or wrong morally, the law still states that taking the snake would be stealing.
So unless you get a warrant or some legal document to give you safe passage, I wouldn't steal the snake.... Speak to a reptile rescue. Perhaps they can pull some legal strings for you.
As someone else has stated, stealing the snake may bite you in the but later on...Just be cautious of the consequences of stealing.
What if your friend finds out and contacts the authorities? The chances are that they order you give the snake back. And at the point, your friend may never listen or let you near the snake again....
In the mean time, if your friend is being stubborn about the snake, donate some temporary supplies or money. Just work with her. Educate her. Tell her what she's doing is wrong.
Maybe she'll have an epiphany along the way..... Show her pictures of what a healthy snake is suppose to look at and compare it to her dying one. If she doesn't understand, show pictures of emaciated dogs. Tell her that is the dog equivalent of her snake since she apparently seems like a dog lover.