Polyamory is not for everyone. And in any case, should not be considered under the current circumstances when you are still actively grieving the loss of your child.
As for your husband, while I can't defend his actions, I can see how this woman sucked him in. Grief is an emotion that people would do almost anything to have lessened. I'm sure that when he thought about her, it decreased his thoughts about the grief somewhat. This is something that she could do that you could not, because as you said, you both shared that loss. When he talks to you about his own grief, there is no relief because you are grieving too. However, this woman took advantage of that, and that is truly horrible.
I think when he began confiding in her, his intentions were purely innocent. Someone to talk to with an outside perspective. In most cases it would not have even been an issue and may have helped him, which would have helped you both. Unfortunately, the woman turned out to be a greedy whack job.