If you go about it saying "You have to stop spending xyz" then you won't be successful.  It maybe why you're getting the "get a better job" defensive comment back.

You've got to bring her into the overall plan and why you need to spend only a certain amount of money.  Now, you need to allow her to have input into the various categories and work with her to get to the right numbers at the end.  Maybe she has the idea to get a job herself because she shes the stuff she wants in the budget and it's necessary to get there.

If you don't connect to the "why", then it will be tough.