there's no need to stomp on XR-7 for participating in a morally ambiguous relationship. there's no (technically) right or wrong way to approach this - if XR-7 did things differently it could possibly have not led him down the path that is now connecting them. there's also no point in taking the moral high ground. nearly all relationships have some morally ambiguous part and if you get fixated on that you won't ever be able to enjoy a relationship.
there isn't much ambiguity. he is willingly participating in the act of cheating on a significant other. I'd say her "usage" of the (now-ex?) bf is ambiguous. but not XR's actions
your last sentence is also ridiculously questionable
context is everything. stop dismissing her from being someone worth going for simply because she broke up with someone else in the process. we can't really know their relationship/friendship well enough to judge its "quality" through XR-7's posts.
I'm not dismissing her, or calling their relationship low-quality
what I'm saying is that he's being a bit of a richard. if he were in the other guy's position, I can't imagine he (or anyone else on this forum) would think it was right. you must also remember that we're hearing all of this through a filter: the filter of the guy who really wants to be with this girl, and probably cares about how other people perceive him