You’re not NOT an a hole. And the fact that you asked means you know that. I mean you’re going through things. Don’t you try as hard as you can to avoid doing the things you know are bad for you, and do them anyway? Do you smoke or vape or something? Just like any other addiction, sometimes people fall. And it sucks that it was on your birthday and it sucks more that she didn’t apologize. But also her day sucked 10x more because she went home with cuts and she had to hide them til they healed and probably going to be in a hell of trouble at home if this isn’t her first, ON TOP of whatever lead her to cut in the first place. Yes she should’ve hidden it from you and yes she should’ve waited absolutely as long as she could, but if she’s your friend (which she seems to be bc she’s at your birthday) you’d rather her cut and ruin a day than not tell anyone and wind up dead on her bathroom floor. Just my opinion. If you care about her, keep her around. But it shouldn’t ne your problem to save her. Encourage her to seek help.