I'm going to slut shame someone I know
You can't do that! You're gonna slut shame her to her face?
I'll tell her that I know that she slept with this guy or that guy
Slut shaming seems like you're way too invested in someone's private life. Leave her alone if you don't have an actual bone to pick with her.
It's fine, she's doing wrong and she should know that.
While sleeping around is bad, there's no need to make her feel so ashamed for it. It's not your problem on what she does, so I believe it's best to just leave her alone instead of attacking her. Or how about talking to her about her actions, maybe she'd reconsider?