*puts on flame suit* Okay - I suspect I'm going to catch a lot of shit for this, but I think somebody ought to provide an alternate POV here.
Get off your high horses. Seriously? In the case of DP and her V with her husband and M - she's been honest with you guys about her other relationship. Should be be striaight with him? Of course! Is it any of your business what she does with that relationship? Well - I'll answer that one with another questio - if she told you that you ought not to spend time with your hubby until you stopped some behaviour of yours that might be deemed destructive, how would that go over?
Your husband is letting her let her own mistakes. He's the one "dating" her from the sound of it you're not involved with her (Yes, you should have some input in the relationship, etc and so forth - everyone has their own style of setting the ground rules for these things we don't need to cover it all here), and the only reason that there is stress in the relationship on this issue is because - as you put it "once removed" - you don't like her decisions.
Why am I the only one who thinks that is kind of ridiculous?
Violet brought up our situation, so let's take a look at that and make sure I'm not being a hypocrite (I'm very careful about such things, lol). Vi and Anne and I have a full triad, and equality is a big deal to us. It bothers Violet and I that Anne is dragging her feet with her 600 mile away guy, we'll call him Mike. It bothers us that she's not open with him - because it affects her being open with us. It bothers us that he treats her like crap, because we have to deal with the roller coaster he puts her on when he treats her like gold. There are a lot of things about her thing with Mike that bother us. But we haven't changed our situation with her one bit over it. Stressed about it? Yessiree. Worried about, talked a bout it, dealt with jealousy issues and whatnot over it - absofuckinlutely.
But never told her what to do. Never. Not our relationship, not our problem. As long as we feel she is open and honest with us, wht she does with Mike is ultimately up to her. We'll offer advice when asked for, we'll offer support when needed.
Heinlein - always an appropriate example somewhere in his books, lol. I'll paraphrase here, because the precise quote escapes me...
"Always trying to make rules and pass laws for what the other guy is doing. Never see anybody saying 'please pass this law to restrict me from doing this thing that I shoudn't be doing because it's bad for me'".