What are you asking her to do? Stay on in an untenable situation? News flash for you: when you make a mistake signing on with a company, you aren't "honor-bound" to stay there until the situation plays itself out. If your situation sucks, leave. She did.

There's another unfortunate lesson here. When people threaten to quit, counteroffers are often a waste of time. People with MBAs will tell you not to make them at all. I've learned this the hard way; I got a really good offer in the middle of my last PM job, brought it up with my current company, and they upped my stake. But for the next 2 years, they were constantly on the defensive assuming I was going to quit again; it damaged the working relationship severely.

If you're a solid company, and someone gives you an ultimatum, a no-foul clean break is often going to be the right move. I wish it wasn't like that (I had no intention of quitting), but some political situations are untenable.