Water ingress can occur from several different areas, so it may take some investigation to get to the bottom of it. Window, boot and door seals can be the culprit, as well as drainage channels, particularly if your daughter's car is fitted with a sunroof. It is also possible that the drain pipe for the air conditioning has become blocked which is causing water to enter the cabin. If you cannot find the source yourself, you may wish to take it to an independent Citroen specialist who may have seen this issue before. You can find one using the Honest John Good Garage Guide here: good-garage-guide.honestjohn.co.uk/