I have discussed things a little with D already. The main problem is, we cannot be sure that she would not have disclosed the other partner before sex with him, because sex did not happen. All they had was manual contact. It's quite possible that she would have told him about the other guy after it looked like intercourse, or oral, was definitely about to happen. That's too late in the game in my book, but I think it still falls within the confines of appropriate disclosure. As it was, they touched each other genitally, he turned down more, and then she disclosed the other partner.
D also has a selective memory about some things that R has told him. He insists that she never specifically said that she didn't want to date men back when she put their relationship on indefinite hold. I am certain she did say that. I think he wants to exonerate her because he really wants a relationship with her. I think convincing him it would be a bad idea would be really hard.