We stayed at the Aloft and LeMeridien right across from the main Munich train station. Both were nice and not that expensive. Train to the airport in the morning.

At the airport, the Moxy is nice and cheap. They have a shuttle that runs to/from the airport, so he could just take the train to the airport, then the shuttle to the Moxy.
Personally, I'd stay near the main train station and go in in the morning.