Luke 16:18 reads “Whosoever putteth away his wife, and marrieth another, committeth adultery: and whosoever marrieth her that is put away from her husband committeth adultery.”
This verse envisions a man Fred who divorces his wife Ethel unscripturally, and then marries another. The verse goes on to say that it is adultery if someone then marries Ethel after Fred has put her away, and even after Fred has remarried and committed adultery against Ethel. At this point Ethel would have scriptural grounds, but the verse still says she may not remarry.
Let’s call “Whosoever putteth away his wife” the ‘a’ part of the verse, “and marrieth another, committeth adultery” the ‘b’ part, and “whosoever marrieth her that is put away from her husband committeth adultery” the ‘c’ part.
We are not saying the ‘c’ part is only true when both ‘a’ and ‘b’ have happened. What we are saying is that it is false that the ‘c’ part is only true when the ‘a’ part is true and the ‘b’ part is false. That’s the “mental divorce” position and is absurd on the face of it.
This verse proves it matters who gets the divorce along with who commits fornication. If it didn’t matter who got the divorce, then the woman of the ‘c’ part ought to be able to remarry, since when ‘b’ is true, she has the grounds. But ‘c’ says she can’t remarry even if ‘b’ is true. It is because she didn’t divorce her husband for his fornication as Matt 19:9a requires.
So you see it really does matter who actually gets the divorce.