She just wanted to do something nice for you, let her.
Sometimes its not about "paying" its about giving the gift of giving.
If it comes to something expensive, like drinks or dinner, and you really feel like she shouldn't pay for you, just say "Ok, you can buy my meal, but only if I get to buy you yours"
If she really does work her ass off for some cash, than it means a lot more to her to be able to treat you to something, so just accept thats what she decided to do, and a simple "Thank you" will suffice.
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