May I ask how long your wife has been involved with this particular dom? If it is a rather new relationship, she might be experiencing what some experienced BDSM folks call 'subfrenzy'. (I don't consider myself one of those 'experienced BDSM folks' btw.) It is similar to what poly folks call NRE or new relationship energy. NRE and subfrenzy can cause folks to make very stupid decisions, ignore prior relationships, ignore common sense and generally make asses out of themselves. Subfrenzy in particular can cause subs to seek out more and more extreme stimulation, more extreme scenes, ignore safety protocols, ignore their own good sense and instincts in choosing play partners. I suggest you go on fetlife.com (I realize looking at a BDSM website is the last thing you want to do!) and look around. There is some interesting stuff on subfrenzy as well as on being a sub in general. It cannot hurt to get some perspective on s
I mention this not to excuse her actions, but to give you another perspective on possible 'why's'.
It is very disturbing that she is ignoring your requests and encouraging her dom to do the same. It is very disturbing that he knows you are not happy about this situation and continues to do what he is doing. A respectful dom - one who gives a shit about his subs and their whole lives, not just their part in his - would have at least gotten more information from you and your wife.
It is possible you may have to threaten to end the relationship to get her attention. It is possible that in the throes of subfrenzy she may decide to end her marriage with you. Unfortunately, it seems to me you are already at the decision point of staying in the marriage or not. And that is something you can control. You can't control her decisions, her actions, or her beliefs. I wish you the best.