I agree with Woozer about the "holding" insurer being obliged to offer the minimum cover required by law.
With regards to whether or not your daughter's partner was insured, you need to read the Certificate of Motoring Insurance. Is he named on that? If not then I presume you are relying on the "any driver" type of policy. However, there are usually restrictions with this type of cover and you would need to read the certificate and the policy document very carefully.
When dealing with insurance companies or insurance brokers it is important that you confirm everything in writing and make sure that they reply in writing. As the absolute minimum, if making contact by phone, make sure you get the full name of the person you speak to and make a note of the date and time of the conversation. I always ask if the conversation is being recorded, so you know they can access it in the future. I know it seems like a pain but it's worth it in the long run.