I feel the need to add my contribution, because I don't agree with Jennifer about giving your seal of approval on creating friendship with his ex and her husband. Probably her husband wouldn't like it either, so you'll have an ally in saying NO. I would say your husband is out-of-control from grief, and his outreach to his ex was for comfort, probably also because she had had friendship with his best friend too. He's in the chaotic phase of his grief response, and intense anger is one of the feelings that are normal for him to have, and to turn that towards you is just a sign of his extreme vulnerability. While you're apologizing to him for contacting his ex, please let him know that you forgive him for getting angry at you for writing her--since you were just trying to stabilize your marriage by not adding unnecessary ghostly feelings from his past to the chaos of emotions you're both going through in the wake of his great loss. He apparently hasn't kept up a periodic contact with her before now, so there's no need to invite more unwanted guests into your marriage.
I agree with Jennifer that right and wrong aren't the appropriate attitudes to think about. But after any significant communications about their grief over the friend, they will only cause discomfort in your and her marriage if they build up a new relationship. It's best treated as a "greeting-card" relationship with the mutual well-wishing and respect for each other as well understood, but only special occasions (like this death) as possibly a real need to communicate. It sounds like his ex is very clear about that already. So you can get his anger to subside by showing that you respect it as you have already felt so much empathy in his grief.
Extreme Grief can push a couple apart or bring them together. And you just want to stop this temporary push-apart by respecting his grief and accepting his anger (which he's denying, but your acceptance via apologizing will work fine). After all, you're feeling grief too, WITH him, so you're emotionally unstable, underslept and vulnerable too. YOU NEED your closeness with him--but not with her. Nor does she NEED emotional closeness with either him or you. So let your husband know again how much you feel in this grief-storm too, and how much you need your closeness with him (that's what your question is hurting about).
I'm glad you're recovering your balance somewhat. It's no surprise to me that your husband wouldn't realize there could be old feelings locked up inside that his ex is aware of and he is NOT--because very few men pay enough attention to their feelings to realize what's lodged inside that they just assumed was long gone down the river of time. In fact I use an image when I teach about recovering from the end of a significant love relationship that his situation fits very well:
No matter how well we try to grieve consciously after a love has gone (or after a death) (and men almost NEVER grieve consciously for more than a month or two, which is not very much, and that's why many cultures have "Memorial Day"s and anniversaries of deaths and lost-loves can be remembrance days. So picture the mass of good, bad, and sad feelings as a Native American burial mound (we have them in Florida above ground). You've let topsoil and some plants grow over the pile of your feelings for a parent, sibling, or el-love that is now gone from your life. But each time that you make contact with her (I'm using your husband as my subject now), you're unwittingly sticking your shovel of conscious attention into the side of your little hillock/mound of soil-covered feeling-memories. What you talk about is what you have pulled out on your shovel, and there might be nothing intense on it. Except in this case there's lots of intense feelings, because his conversation is about love and loss, of his best friend, and with his first "best love." That emotional intensity itself will bring out more intense feelings from his past with her that were much more buried. Or visualize that this way: When he pulls out a shovelfull from the side of his feeling-memory mound, there will be a small landslide of the topsoil above where his shovel went in. And that landslide will slide away from several outcroppings of feeling-memories that have not been noticed for many years. That's what has happened for her, and that's what she wrote to you--even though he's not paying enough attention to notice that he might have some old feelings sticking out too.
The good side of this is that when those old feelings come into consciousness in this way, they can give rise to some conscious thought, and that thinking can revise them in the context of his present life, so they are modified and reduced, which ultimately reduces the mass of the hillock/mound itself--though in principle, NO past-relation-burial-mound will ever become empty and part of a flat plain. By thinking about our past feelings each time they happen to come to the surface, we can revise them, redigest them, and revise who we are, as well as our relationship to all of our loves, both present and past.
So you could suggest to your husband that he write FOR HIMSELF, never to show or tell to you, about what his thoughts about his ex have been, as well as PARTICULARLY about his thoughts about his best friend. ("Thoughts" works much better for men than "feelings" as a name for this journaling, though every "feeling" has thoughts imbedded, and every "thought" is colored by its feelings.) This aspect of grieving is very valuable for everybody, and most people don't do it at all. I'd advise you to write about your feeling-thoughts about your former love(s) if any have come up. DREAMS are also very helpful at such times, because they contain symbolic guidance from higher wisdom (from Higher Power, or for churchgoers, from God) expressed thru our unconscious projection system of images, story-plots, words, sensations, feelings, and thoughts, whose aim is to show us how to gain new wisdom from our contact with the departed (or "lost love") so that our continued living can be richer for our conscientious honoring communication with the departed.
If it should please you or your husband to communicate with me about any of these options (at the very low fees this website charges), I will be happy to respond. I believe you already paid Jenhelant for her answer, so I'm not asking you to relist and pay me anything. For it is my legacy to offer learning about such a situation as you are experiencing, and tomorrow is my 70th birthday.
Thank you for the opportunity to think about your transition,
Norman Brown, PhD, LMarriage & Family Therapist, moving to Atlanta this month.