we'd really like to know whether their flight is one of the ' hundreds of flights cancelled ' according to UK newspapers before we set off.
The strike in Marseilles was announced weeks ago ; 23-24 June .
ATC does not cancel flights, just causing delays ,as not all the sectors are opened. there is also a minimum service mandatory for ATC in France during periods of industrial action. It is the airlines that cancel flights , so in your case call the airline . they know.
Finally Newspapers ( especially UK ones except perhaps The Guardian ) are rarely a good source to find something in Aviation ..