This is a morality question more than anything... but technically you are not involved in this scenario except on the periphery, and telling on your friend to their partner is going to be seen as interference at the mildest and downright rude and obtrusive on the other end.
I do agree that your friend should be informing their partners... but what they are doing is usually not considered illegal as herpes is not a fatal illness... so you wouldn't have legal grounds to interfere here.
And in reality, what are you going to do... follow her around and tell anyone you see her hitting on? I understand your feelings here.. and it is wrong for her not to inform her partners... but it frankly isn't your business to get in the middle of to begin with... and if you do and your friend finds out, the best that will happen is she will kick you out of her life and lie to her partners if you come around again.
And lets be honest... if someone wants to have sex and sees her as a possibility, even if you tell them, they are going to go to her, she lies, and they have sex anyway.
Best in this situation to put as much space between you and her as possible. If you feel like your conscience is going to burn at you for not telling, then for your own sake, but realize that in all likelihood you are just gonna piss off both her and whomever is queuing up to be next on the log flume, and they will do it anyway. Cause they are young guys with only enough blood for one brain at a time.. and she is easy.
You can't be an eternal guardian... make the break and run for the hills.