Hi LadyJ....
I think her husband is correct.
If he names her on his insurance, she will be covered in the same way he will be covered.
If he is fully comprehensive, she will be also.
This means if she hits someone, the insurance will pay for the damage, but she will have to pay the excess amount in ££.
The bigger the excess amount, the less chance people will claim for smaller damage.
If she goes on her husband's insurance, she will not be able to build up her no-claims bonus, which is measured in percentage - 40%, 60% etc.
This is how I understand it, but she should always consult many different insurance companies and ask them to explain their offers.
I hope I am correct here and I hope it helps.