I suppose that he has discussed this with your daughter and she has said that she wants to see the friend. That would be a manipulative thing to do, so I expect that to be the situation.
It's up to you. If you say anything other than what you want to say, you will feel bad all weekend. If you say no and D already knows, and thinks you are mean, well, people need to learn that they will not always get what they want. It's never too early to learn that. And if she doesn't know, then there are no other ramifications.
I think it's worthwhile saying no and then seeing if he honors your wishes. It gives you some insight into him.
