I'm not sure why she'd have to go through immigration/passport control at Munich, since she will be in transit between one non-Schengen country and another non-Schengen country. As far as I know, German airports (unlike US airports, generally) are set up to accommodate international transit passengers without them having to enter the territory.

Or am I wrong somewhere?

She might well have to show her passport to the airline, and herein might lie the problem on her way back to the USA, since the airline will want to check that she has the right to enter the USA. (I can't see any problem on trip to Romania.)