Of course it depends on the country, but by EU rules, they would need to apply for an exemption from the rule. Which would need to be short notice sickness of a crew member that could not be replaced, or a similar type of reasons. And while they are pretty standard to get, I can't imagine FR. filing such exemptions without a better reason than we scheduled a bigger plane than planned, or we did not schedule crew for the size of the plane in the first place, as that certainly would be an exemption reasons.