In Model Builder just add a 'Calculate Value' item to the model ( Insert -> Model Only Tools -> Calculate Value ).
Make sure this runs after all the parts of the model that affect map layers have completed by making one or more of them a precondition of the 'Calculate Value' item.
Open the 'Calculate Value' item and enter the Expression: `arcpy.RefreshActiveView()`
The result of this calculate value item will actually be meaningless, but it will refresh the map.
General Note
The 'Calculate Value' tool in Model Builder is designed for using a script (or single line of code) to calculate some value to be used elsewhere in the model.
But you can ignore its output, and it turns out that it's really handy for simply running snippets of Python code to do anything in a model that you can do with Python that can't be easily done with other tools.
I would never have thought of this totally obscure method. Seems to work well for me. And the idea of using this to run oher snippets of code within a model is something I will have to look into. Thanks to TasmanianIrrigation from sunny Brisbane!