Just to be realistic here with time requirements. We suppose the incoming flight is not late by a minute and we suppose that he has to be there no more than 30 minutes before departure to Venice. If there are 4 hours between these two points on the timeline, he will have time to do the little walking tour suggested by Reindeerflame or Flying Lawyer.

To be on the safe side from the moment you touch down to the moment you get off the train at Friedrichstrasse I'd count 1h15min. To get back from the moment you take the train from FS to the departure terminal I'd count around the same. This includes some waiting for the train and orientation at the airport and the S-Bahn station. This leaves about 1.5 hours of walkaround time in Berlin.

If the premises in the first paragraph are not met, I wouldn't bother. Too stressful and too risky to miss the connecting flight.

Till