gm...hmmm, I hope I read your question correctly. This is one of those questions that would be better asked of the "unrelenting women," you mention.
However, I will assume you want our best guess.
Because you use the term, "relationship," rather than "married," you make this much easier for me. LOL "All's fair in love and war." is not actually accurate......because there certainly ARE things which are quite UNfair in both love and war....so it's fair to say, I don't agree with that statement. We need to use our own belief system and concept of morality.
These women who are supposedly "willing to do anything to be in a relationship," are everywhere....and in multitudes. If it's simply a relationship...."any relationship," there doesn't seem to be any justification to set their intentions on a man who is already in a committed relationship...when there are plenty of unattached men. Additionally, I have no way to know what you mean by "willing to do ANYTHING." "Anything" covers an awful lot....and some, rather gruesome to consider.....
Getting back to these conniving women.....If it is a particular MAN they desire....and that man is currently involved with another woman....I see no reason why Woman B cannot place her bid (LOL) as in, allow the man to be aware of her feelings and desire for him. Ultimately, it's HIS choice, now isn't it?