The guy told a story. The story had holes in it. When looked at through these holes, the story kinda makes his girlfriend look like an over-reacting harpy. That's funny. When you consider that it's practically fucking impossible for the cashier to have perpetrated this genius scam, the following two sentences...

"The cunt working the counter overcharged her a tip. I'm no nickle dimer or cheap ass, but this was theft, plain and simple. My G/F went back to complain and they offered to give her a buck, but never a word about disciplining/firing the employee."

...are hilarious. Add to that the total non-sequitur...

"I told her to find the manager of that store's manager and let him/her in on the scam, but maybe that's because I can't fucking stand how the cashiers at Starbucks insist on being called "barristas". They're fucking cashiers."

...and then the cunning revenge scheme...

"My second suggestion was to find out what car was hers so I could gouge a key scratch in her ride that would negate not only my girlfriend's stolen dollar, but hopefully every other dollar she's stolen."

...and you've got one hell of a misguided rant. Even when posed with the notion that maybe he and his lady were mistaken, he was more than happy to remain outraged at Starbucks. He didn't care that he was wrong, he wanted to be owed. He wants people to see that he's been wronged, even when he's shown that he hasn't been wronged. He wanted to complain, be heard, and be validated.

That folly, to me, is funny.