It suddenly hit me that Brian and Gabby may have had an argument in MP, something that had nothing to do with the food or service or the restaurant at all. That could be what the witness overheard. Then, in the heat of the argument, the two walked out, perhaps Gabby leaving and Brian chasing after her or vice versa. The waitress realized they hadn't paid, followed them out and they paid. Then, perhaps they hadn't eaten and they re-entered the MP to ask for a refund for what they hadn't eaten.
The story we get from the manager may have only been part of the story. Just as what the witness stated is only part of the story.