I don't understand why you think telling her would be cruel or punishing her.
Option one: you tell her "I'm going to cut you out of my life", explain why, and cut her out of your life. She knows where she is standing.
Option two: she stops seeing you around, might ask about you in case she's worried something happened to you, and eventually understand you're avoiding her. In this case, probably understands why, but is always left with the fact that you did even give her the respect to let her know about it.
I've had lots of people cut me out of their lives after I broke up with Raga. I lost all of my family on his side (so, his family), plus a bunch of friends we had in common. Almost every one. And each time, the person never let me know. And it was really tough. At first, I thought people were busy supporting Raga through the breakup so I didn't insist.
Then I started noticing some people hadn't updated their facebook statuses in a long time. I tried to go on their wall to check on them and they weren't in my list. I looked through my list and noticed a bunch of people missing.
At that point, I understood people were upset with me. I had no clue why, though (of course in your situation, chances are she would).
As far as I knew at that point, Raga and I had left on the best of terms. The last things he had said to me was that he wanted to stay friends, that I was still his best friend, etc. After that I had given him room to heal and so on, so I had no clue what had changed.
Only later did I realise a few things. One, Seamus had finally posted pictures of us together from 4 months or so prior, as we had waited for me and Raga to have been separated for a month first. I knew he'd be linked to them (as they were on each other's friend lists) but Raga had seen these pictures bunch of times and I didn't think it would be an issue. Turns out all my contacts also saw them because despite not being the one who posted them, I was identified on them.
At that point I though, did people think I was cheating? Raga didn't want us to be out, as much as I wanted to, and I had always respected that. I had assumed he'd have told them by now though, so I was surprised.
At that point, only people who didn't know we were poly seemed to have shunned me.
Later though, things like that happened again. I saw someone online on a chat program, said hi, and saw them get offline immediately, never to reappear online. On one occasion, the guy first told me "I think what you did to Raga is horrible and you're a terrible person. I never want anything to do with you again" and then blocked me before I had time to ask what it was that I did.
Only one person sent me an email, months after the fact, when I sent her one to ask if she was doing okay and how I was worried from not hearing from her. I was surprised that she was shunning me as well, since we were friends before she was friends with Raga, and I had always assumed our friendships would both continue independently regardless of whether I still talked with Raga or not.
If they had told me about it, it would have been much, much less hard. Having to realise it for every person again was like being stabbed in the back, and the fact that none of them talked to me first was the worst. They never checked with me what was true and what wasn't. They never asked for my version of the facts. How could I have thought they were my friends all that times? Friends don't judge before they have the facts. It was really harsh.
To this day I wonder how many of them would still be my friend if they had asked about my side of the story, and I have problem thinking that the answer would be anything but "all of them".
I think you should tell her how you feel. Your situation is different and unique of course, but right now it seems you know of everything second or third-hand. Even if you're sure you will act the same way, it seems to me you owe her to show that you are listening to her side. Otherwise, what if next people say your husband abused someone and that you're still with him, and they all shun you even though none of it is true? Not saying that it isn't true, but that I think you should hear it from her before you judge her for it.
Communication is important. If you weren't there, I don't think you can make a decision based on what other people say they heard. Maybe she needs help. Maybe she's coerced. Maybe she did make that decision. But in the end you don't know for sure until she's the one telling you about it.
And to send her an email and read her response, you don't need to risk your children's safety in any way. So it seems to me not doing so isn't anything other than cowardly.