But we are told that Gabby went back in with Brian and she asked for their money back. This indicates some level of dissatisfaction with their food and/or service. It also shows Gabby was on board with whatever their reason was for not paying.
Frankly, the manager's story really looks like an attempt to minimize what really happened. If “there were no loud voices, no one got angry and no fit was thrown," then how did Nina and her boyfriend even notice that something was going on?
Nina: "My first thought was that they were displeased with the service,” she said, “and they were walking out or they were trying to get out of paying their bill, but it was a scene."
When the manager says they were trying to leave without paying that doesn't necessarily mean they tried to sneak out without paying. It sounds like they said, "we're not paying" and walked out, with the waitress following them out and demanding they pay. Eventually they relented and paid. But they still felt for whatever reason that they shouldn't have to pay.
Source for most of above:
Online Sleuths Are Skeptical Of Wyoming Restaurant Manager’s Account Of Night Gabby Petito Was Last Seen
What I do find a little off in all of this is that it doesn't sound like Brian and Gabby were actually fighting with each other. It was both of them vs the restaurant. So how did this eventually lead to Gabby being murdered? Could it be completely irrelevant? Does it merely show that one and/or both of them was in some kind of bad mental state?