Get a computer that supports two monitors. Turn one towards the customer. With scripting, you can put up one window on the customer monitor and a different window with a different layout on the sales person's monitor if that is desirable.
If you want them to see exactly the same layout and data, this becomes very easy as most computers that support more than one monitor have an option that simply copies the same output to both monitors.
Note: on windows machines where you wan to use different windows, you have to open the FileMaker window on one monitor and then drag the edges of this window to "stretch" it onto the second monitor so that you have one "application window" stretched over both monitors. Then you have to script carefully to size and position FileMaker windows inside this one application window.