I have been following French rules very carefully and there has never been a time that France admitted EU but not non-EU citizens travelling from the UK, unless the EU citizen was on their way to their country of nationality or residence. They always allowed French citizens in, as they had to, but I and other non-French EU citizens living in the UK were subject to the same rules as UK citizens. As such, it seems to me the easyJet staff were correct.

If the person in question was allowed to travel to France on Eurostar, it will have been in error, or based on an assumption that she was travelling onwards to the Netherlands. Due to the layout of St Pancras station's departures hall, passengers are not normally separated by destination.