Originally Posted by
osmantrex
...
They took the first leg of the itinerary back to the states, ...
This is the only important point. This is a partially flown credit. Basicly you have an open ticket credit, not a general credit. Your credit is still governed by the original ticket rules
Originally Posted by
osmantrex
...
but that also the ticket must be used for an itinerary between Denver and Europe. ...
likely correct.
UA will not convert to an ETC
There is not much you can do. Find the absolutely cheapest return flight and you may get some credit in the form of a general FFC (this may not work due to the OTA booking -- there is no residual credit for lower priced rebookings for OTA tickets.)