If you are in sales, your best friend or your worst enemy is the SECRETARY at any business you are trying to do business with. If you befriend them, they will push past the rules and get you an appointment with untouchable people. Tick them off and you won't even get to people who WANT to see you. To someone walking in from the street they are more powerful than the CEO. They can be a red carpet or a brick wall.
In a bar, the bartenders/wait staff wields the same power, if not more. Your relationship with them (9 times out of 10) will matter more than even how much beer you sell or how many people come to your shows. If they don't like you, you ain't coming back. Period. If they love you, often the manager will find a way to bring you back even if your numbers aren't amazing.
If the place is casual, and you are FRIENDS with people behind the bar, no big deal. There are lots of places we play where I can yell "Hey Barbara! Bring me a beer woman!" over the mic. (And she's the co-owner with her husband.) But we have a history. We're friends. Heck, I can stay at their house on the beach after the show if I want.
There are other places where I don't know anyone. They are a bit more upscale. I wouldn't even be caught with a beer on stage if I paid for it. I don't know how they feel about those things yet and I'm on stage with other guys who count on these gigs. I do a lot of fill-ins so I may even be a hired gun that night. I wouldn't DARE do anything to hurt their reputation.
If the guitar player and staff are total strangers, or this is a more upscale place, tell him to shut up........ now.
Like most questions, this one doesn't have one answer. It's called being grown. You act appropriately for the individual situation.