non EU family member from France travel to UK

[AlfaBeta123]

Hi all,
Hope someone can advise.

My sister in law is a non-EU family member (who holds a Residence card (French issued)), who is married to a French Citizen (EU National). She wants to visit the UK for a short visit to travel with her spouse and back with her spouse).
If she takes a valid passport, marriage certificate and travel with her EU/French Spouse does she need a visa or can she travel with the above mentioned documents to the UK?
hope you are able to advise.

[secret.simon]

How is she travelling? If by flight, the airline may refuse her boarding as she does not have a visa to enter the UK.

She may have better luck if she travels by Eurostar or car. That way she can deal directly with the UK Border Force at the relevant railway stations or at Calais (if going by car).