In order to be successful if is imperative to be customer focused, yet employee aware at the same time. However, at times, these two important groups may come at odds which is when you as a leader need to act. Ideally, you would have been involved all along to avoid any negative escalation of issues, challenges or problems, but that is a story for another time.
When someone attacks your team members you must educate yourself on what has happened and if no fault can be found you need to respectfully stand by your staff. Leaders must step up and provide safe environment for their teams no matter who might be on the other side of the table or how loud they get. Leadership must clearly demonstrate commitment towards conflict resolution and support for staff alongside zero tolerance for any type of abuse by anyone at any time.
After all, this is your team, support them.