I answered this more in-depth in your other post, but just focusing on this specific question for a moment....
I think it might help to break it down:
The first part is him and her and their agreement. That's completely between the two of them (and I mean that genuinely - not tritely).
If she doesn't want to know anything, that's not only her choice, but her right. If it works for her, it works for her. We can't judge it, because she could have a myriad of reasons for this - and those reasons could be good ones.
So - part 1 is: what is their agreement?
If he's not telling her because they've agreed not to talk about it, let it go. He's only honouring her wishes.
If they don't have a stone agreement, they need to make one.
If she has asked, or he can tell her, he should tell her. If he's avoiding it because he's being a communication chicken, rather than concealing it because he's honouring her wishes, it's a lie of omission.
My GF and I have this phrase that has gone around and around in our relationship: "It is what it is." That means, if you love someone, you love them. If you love them and I'm less important, make that clear to me. If I am more important, make that clear. If I'm equal, make that clear. If you want to spend your life with me, tell me. If you want to share it with someone else now, tell me. If sex with me bores you, but you love cuddling with me, tell me. If sex with me rocks your world, tell me that too
That doesn't mean that *I* have to know every tiny little detail about how every little action or thought makes my partner feel. It just means that I want to live in the real world and make my own decisions based on the facts. To me, love, commitment and sexual preference are important facts in a relationship. I don't want to waste time and energy investing in a relationship that I am more into than my partner is. So, give it to me straight, on those three key things (the levels of sexual interest, commitment and love). I'll decide what to do from there.
So, that's D and R's relationship. If he's honouring her wishes, great. If he's being a chicken, not great.
What do you want? What does he want? In terms of poly communication stuff.
Get some guidelines written down, if you haven't already. Even if your only guideline is "tell me if you love someone" (or whatever it might be that's important to you).
If he can't, or doesn't want to, agree to that, you have a compatibility issue.
So often, we think we've made an agreement (i.e. your previous request to know if he loves someone) and it gets lost in all the other little day to day chats. If you write things down, each have a copy and read through them every three months, you're unlikely to forget about their importance.
There is one other thing that might be useful for you to think about. Are you happy and fulfilled in this relationship? That's not a leading question; it's genuinely an open one. We can't always get absolutely everything we want - it's often about compromise. But something struck me about your thoughts.
I am similar to you in that I want and expect all partners to know about each other, know where they stand, know what the sexual, love, commitment levels are at. I want that because I think it makes realistic sense. But, there is a part of me that also wants my GF's levels towards me expressed to others, because I've worked hard to earn this place in her life and she's worked hard to find her place in mine. I'd feel disrespectful towards her if I pretended she wasn't as important as she is. It's also important for me to have a relationship where I am seen as a priority, or as being very important. Some people don't care about that - I do care. I'm happy to be a secondary partner to ten different women, providing I have one primary partner who I am important to. That's just what I want. What about you?