The Destructive Nature of Rumor Mills

From the Bible... "A perverse man stirs up dissension, and a gossip separates close friends." Proverbs 16:28. Apparently gossip has been around for many millennium.

The setting: Michael Emerson is leaving Notre Dame. The Notre Dame department hasn't announced it yet because they are trying to find a new director for his center; that way when they announce Michael is leaving, it will soften the blow by saying, "But it's not so bad, because we have Dr. X to be director of the center."
I've known about it for awhile; since there's no one else at Notre Dame who studies race, I've had to apply to transfer to another program for next year.

The situation: My friend Brandy emailed me a few weeks ago asking me to cat sit for the summer. I just avoided responding at all. I won't stay in the Bend for the summer if I'm not staying at Notre Dame, but I have tried to not straight out lie to anyone. Brandy tracked me down in my office the other day. I decided I had to tell her the truth.

Valerie: I have something to tell you Brandy.
Brandy: Okay...
Valerie: It's a secret.
Brandy: Are you leaving?
Valerie: Yes.
Brandy: Because Michael's leaving?
Valerie: How did you know that?
Brandy: I heard a rumor to that effect from Bill. [Bill is a professor in the dept].
Valerie: Yeah, it's true, but no one knows, it has to be kept quiet for now.

We have a little bit more conversation, whatever.

Next day, I'm sitting in class, and random other girl in our program is like, "Val, I heard a rumor that you and Michael are leaving. Is that true???" Cue Valerie flipping out.
"Where'd you hear that from?"
"Dan and Amanda. I assumed they heard it from you, so I told Rich [another professor] about it, and he got all upset that it couldn't be true."

What??

I directly go and tell Michael the situation, very apologetically. He already knew about it, because someone told someone told someone who told his poor sweet secretary whom he hadn't been able to tell yet. And given we know of at least these two long gossip chains, it seems pretty certain the rest of the department knows. Poor Michael still isn't officially allowed by Notre Dame to tell anyone, so he's having to lie or worm around it or just avoid people. I was so apologetic, but he was like, "No, this isn't your fault, I don't harbor any ill will towards you about this." Still, he is in a really tight/awkward spot.

Unfortunately, I do harbor ill will: towards Brandy. I emailed her about it, and her response was, "It was too good not to tell people." (So apparently she told pretty much anyone she saw.) I'm so mad! So many things were jeopardized by spreading a little gossip: my relationship with Michael, Michael's relationship with many people, reputations...

So, I don't know how to deal with Brandy. She's a friend, so I don't just want to write her off. But this is unacceptable. How do you address such deliberate thoughtlessness (since I told her it needs to be kept quiet) and what I see as selfishness? With a response like, "it's too good not to tell people," it doesn't seem like she's going to be too apologetic about it. Do I have to relegate Brandy to 'sort-of-friend-I-tell-nothing-to'?

Suggestions welcome.