I wouldn't have thought that a flight from Russia would land in a "Schengen" gate at an airport so would have assumed that some sort of immigration control would be in place for such flights.

In any case, in common with every airline, it's the passenger's job to make sure they have any visas/waivers/documents required for their journey, not BA's so if there was any sort of difficulty it would be at the passenger's door to sort it out.