Todd,
Munich to Berlin and back is not really a "day trip". Even using the fastest direct ICE trains, the trip is about 6.5 hours each way. Sitting on a train for 13 hours to spend only a few hours in Berlin will not be a pleasant day (and that doesn't include the time from Berlin Hbf to your friends house and back).
Travel by air isn't much better. The trip from Munich to the airport will be about 45 minutes each way, you'll need to arrive at least 90 minutes prior to your flights in each direction and of course endure the usual hassles of security, waiting, boarding & deplaning, etc. Add to that the flight time and of course time at the Berlin end for travel to and from the airport. The overall time for travel on that route will likely be at least five hours each way and perhaps more.
Some options would be to spend one night in Berlin or perhaps you could meet your friends somewhere in the middle and spend a day with them there. Frankfurt is one possibility as it's about 3.5 hours from both Munich and Berlin (depending on which train you use).