Ninido wrote:
AA has a pt.^

Also, the way the OP worded it - it sounds as if he gave her the $25 BEFORE she gave them the bill. Which I would have believed was for me/her to keep and NOT toward a bill they had not yet received. [And yes, as a former server, I would get handed 20s or whatnot before dropping the bill as a tip.. so it's NOT uncommon at all]
Even if she legitimately got the impression that the money was a tip it was highly unprofessional for her to argue it when she was corrected.  She should have said, "I'm sorry, my mistake.  I'll get your change."