Hi, Steve - I see in your template you do have the 2010 client DLL. But in the error message, it shows the 2013 DLL:
Could not load file or assembly ... Qdabra.Tools.RulesLibrary.Client2013.dll' or one of its dependencies. This assembly is built by a runtime newer than the currently loaded runtime and cannot be loaded.
Did the template at one time have the Client2013 DLL? Is it possible the users w/ this issue have the old template cached? In InfoPath Filler, they can go to the File > New menu, find the template, right click and select Get update or Remove to either update or removed the cached template.