Thank you for your question.
This woman is not doing well in terms of her life, and although she is not a threat to your marriage through physical and/or romantic involvements, an unhealthy emotional enmeshment with your husband seems to be building.
What you are most likely seeing here is a male emotional need being expressed; the need, to rescue a less capable person.
In this case that need is being met through this highly dependent woman. The resistance to talk about it with you usually comes from the man's view that without him, this helpless person will falter and fail and get hurt...and the guilt of letting that happen will often overrides all else.
It is typically the guilt that drives the behavior, as letting her fall apart, which is what is needed to help a dependent personality, is extremely difficult for a strong and capable male to do, which is what your husband seems like.
Anger and defensiveness is a secondary emotion. That is, they cover other emotions. So, guaranteed what he is feeling goes beyond just anger and defensiveness. The saying in psychology is very true. When you see a reaction that is beyond what you would expect, it is a need being expressed that is hidden.
What to do?
No matter what, ultimately he will need to talk with you to resolve this. How to do that?
If he is open to it, a conversation with some preset ground rules often will do the trick. A place, like a restaurant, public but not too much so, that allows you to talk in peace, but also controls the expression of emotions because it is public, is a good place to start.
Typically the rules are that you talk five minutes, then he has two minutes to respond by not expressing his opinion, but summarizing and reflecting back to you what he heard. If he gets that accurate, the role is reversed. The time limit is no more than 30 minutes total, and the conversation must end in that time.
Your husband is probably a very good man who has allowed some needs, maybe from long ago, take over in his interaction with this woman. She most likely elicits feelings of fear that she will be hurt or harmed on his watch, and as a result, he wants to make sure she is okay...for if he does not, who will?
Truth is that she must be allowed to fail. And, he cannot and is not responsible for anyone else but his own behavior.
But first things first. He has hurt you. He may not get that now, but I am confident that he can hear this if he is given a good communication opportunity to do so. He does not strike me as closed off, just tenacious, and I believe the communication style described above is the best way to open those doors.
If that fails then I would consider seeing a counselor face to face to express these feelings, but I also feel that this has an excellent chance to work as he needs to hear you and to turn off his automatic emotional response and this technique allows it. Steven