Practically, the FO on this flight had just completed a type rating course (ZFT) and part of their line training, so in terms of operating the aircraft, would be pretty current and capable, especially as you cover stuff in conversion that you don’t often see in recurrent training and a lot of it would have been NNPs. Risk to aviation: negligible.
Having witnessed the FAA go after a foreign crew that followed procedure approved by their airline, regulator, aircraft manufacturer and the FAA themselves, I would have second thoughts about continuing to the USA once the rostering error had been discovered. I suspect that’s one of the reasons Virgin management asked them to return once the situation became clear.