I hear what you are saying. There's one point that I'd respectfully disagree. I think it does not matter anymore whether she wanted this D/s relationship or whether it was consensual. She made a series of bad choices and deliberately kept his inappropriate advances hidden from her husband. Ending up in that relationship (and it was a long term relationship) was a result of her own choices and decisions. She has only herself to blame for that.yeah, but in the context of a DOM/SUB relationship, all of that is kind of expected.
Turning off the oral sex is a big tell. I am sure there was anal sex provided to the dom too, that was denied the spouse, as well as all sorts of other kinky sex.
Was she coerced into all this, or did she actually want the D/S relationship...who knows? But it was such a lengthy one, it is hard to imagine she was not a very active participant.
I would polygraph her, and ask about all the different types of sex they had, and if the Dom told her to not provide the same to her husband. That would tell the OP for sure what was going on.
If it was not a consensual D/S relationship, maybe she was legitimately blackmailed into it all somehow.
Right now she's doing whatever she can to avoid taking responsibility for all that by playing the victim. She may have been a victim of his devious scheming AND at the same time she was an active perpetrator in cheating on her husband and starving him off her affection while giving it away for cheap to this contractor.. So, if anyone is a victim here it'd be OP. Till she realizes this by herself (or with the help of her IC) all that she shows will be just run-of-the-mill regret that all cheaters have. It most definitely wont be be remorse.