Gift Certificate

We received a gift certificate to a very nice and somewhat expensive restaurant from our daughter. Knowing this is a restaurant that our friends have wanted to go to, we invited them to go to dinner with us with the understanding that the gift certificate would go towards the bill and then we would split the difference. My friend tells me it would not be polite for them to use a gift certificate that was given to us and they should pay the difference up to the amount that matches the gift certificate and then we split the difference after that. Is this correct?