You have a long relationship with this customer and it is exactly at this time they need to know that you're genuinely upset with whatever has developed. You need to take a breath before you engage with customer or staff. You also need to understand that giving up completely to whatever demands are made and make the situation go away is not always necessary.
There's no question that the yelling, screaming, abusive jerks are the worst, and they can be very painful and difficult to deal with. Before we look at them more closely, let's consider the six more subtle types of jerk boss behavior and how to apply to each type the withering medicine of rationality, orderliness and professionalism.