It's been my experience that folks with cutegirl's personality don't understand everyone not being as open as they are. They also tend not to get privacy or keeping things in confidence. I am glad cutegirl is not malicious but - again in my experience - folks like her can be easily manipulated by those who are. I don't know why those personality traits seem to go hand in hand with lack of discretion but they seem to, at least in my life. So I don't tell them anything I wouldn't want the world to know. And I am generally not close to people like that.
You are already close and intertwined personally and professionally. I guess I would prefer a rip the band aid off fast approach personally. Tell her why you are hurt, what her actions did to you. Tell her exactly why you are ramping down or ending the friendship. Don't do it expecting her to change. She won't. She might not even see that her actions were inappropriate. Do it to get your pain and hurt in the open and out of your system. Defriend malicious guy but don't engage further. There is no point in that. If pressed you could say that he showed you who he really is and you are not interested in any interactions with him.
I don't see the point of telling cutegirl's primary. Unless it's a brand new relationship he already knows she is incapable of keeping secrets. You may want to tell him why you are slowing down/ending the friendship with her especially as you and he are friends. But that's more of a 'straight from the horse's mouth' kind of thing.