From what I've read on this so far, for all anyone knows for absolute certain, the credit card info could be just as faked as either of the paper receipts might be.
Given scanning and photoshop capabilities today, the credit card company would have to verify personally their records to either the press (not simply a piece of paper supposedly from them handed over by a 3rd party--and a good reporter would have questioned that) or for a public criminal inquiry about possible fraud should this go that far next, in order to know for certain the veracity of the credit card information.
I tend to believe the customers based on two merely intuitive things about the waitress. She didn't answer directly the question of whether the note was written by the customer, saying instead "I know it isn't my handwriting" which seems a little evasive. Also I wonder if the waitress hadn't been offended by being called the wrong name, taking that out of context and seeking revenge for the assumed offense.
I also kind of thought bullshit that other story about a carrot cake for a different lesbian. They received a homophobic cake but said they'd eat it anyway? So that's either more bullshit or they're just stupid.