ISTFlyer

As most of the folks here have told, it's the responsibility of the passenger who must check that he/she has the required travel documents.
Condor is also responsible for checking them at the first point of departure ( in this specific case: the departure airport at USA )

AFAIK, Condor asks you to type in your passport details during OLCI ( and visa if applicable ) to complete the process. I really wonder if the traveler did check-in online or not? Did he/she succeed checking online if attempted?

Also, when being deported from a country. It's not obliged to pay a return ticket. It's the airline who brought the passenger to that country who is responsible to pay for the expenses, in case of a fault of them not checking travel documents at the first point of departure so the traveler shouldn't have paid Condor for the return ticket. They should have figured it out.

And, the travel agency does not carry any responsibility.