Oh dear, not filing a PIR at the airport does make things messy.
Where is your sister now, can she go back to CDG? If she can, she should go back at a time when Air China would be checking in for a flight, approach check-in and ask to speak to the supervisor who in turn should be able to get hold of the Air China CDG station manager. Your sister probably needs to speak to that individual.

Alternatively if you are near another airport to which Air China flies, the station manager of another outstation can probably contact the CDG team.

Bonne chance !