xjumper--I must say, I don't see a problem. You might take a look at
this book about 'the ethical slut' --your friend may be interested too.
A question though--is she reacting to being called a 'slut' or is she reacting because you're trying to change her behavior? The label 'slut' sounds perfectly accurate for her, though maybe a bit denigrating (like calling someone a 'faggot'). 'Likes to party' might be a nicer way of putting it? If it's a matter of trying to get her to change her behavior--let me ask--is anyone getting hurt by what she's doing? Either herself, her friends, or her partners?