Daughter's college roommate & her mom bought a trip top Santorini through TC.
Santorini is in Greece. Greece is part of Schengen.
They were subsequently denied passage in FRA as they had a day layover. When they were going to leave the airport for the day, they were detained,
This makes little sense.
I assume they were connecting in Frankfurt to Santorini (possibly with Condor via Mykonos), they had to enter the Schengen area here in Frankfurt.
Whether they had a day layover or not - it does not matter. German immigration officers (Bundespolizei) wont let her in.
The German customs officer told them they see up to 20 a day (seems high) and its very routine for them to send people back.
This is also weird. German customs officer (Zoll) has virtually no jurisdiction to send people back. Customs officers are there to check baggage, contraband etc.