Declare an event on the class that has the method that does the reading of the text file. Subscribe to that event using a method on your progress update class. If the method signatures of the event you declare and the progress update class method don't match, you can create an intermediary to handle the conversion. In order to do that, you can attach an anonymous delegate to the text reading event... the anonymous delegate signature must match the event signature. Inside of that anonymous delegate, you can then call the progress update method with whatever parameters it takes.
Whenever you want to issue an update of the progress from the text file reading method, just call the event, passing whatever parameters are necessary (such as percent complete). Just FYI, there is one gotcha you have to be careful with for events... they can only be called from the class that it is declare in.