To not want DDs friend to come around?

[unipolar]

Dd is 14, one of her friends is 15. Whenever he comes here he is quite rude. He also swears a lot and DD has younger siblings.

Yesterday, he came here and I overheard him calling a girl a slag.

WIBU to say he can't come here again?

Maybe have a word with him and set out your boundries and expectations and if he disrespects your rules after that then don't have him back.

Yes

He's her friend.

What you could do, is tell him he's not allowed to swear in your house, he's expected to behave with reasonable civilised manners and that you don't tolerate hate speak like slag or any other dehumanising terms. While he's in your house, he abides by your standards. If he can't do that, he will not be welcome.

State the boundaries. Stick to them.

You may need to have a separate discussion with your DD