I think if there's no hope with dope, then I would need to tell that friend straight up. If they're my friend then I owe it to her to not let her not look stupid in persuing something that really isn't for her. But if I think they can be "molded" then I'd give tips to how they can improve or what they're doing wrong.
I have a friend that just needs some help & I've been trying to help her but in the end it's up to her. I wouldn't try & bad mouth that person but like ladygreek said saying nothing for that friend speaks volumes.