Do not take back one’s divorced wife once she has remarried another man.
“Her first husband who sent her away may not take her again to be his wife after she has been defiled” Devarim 24:4
If the first husband did take her back and was sexually intimate with her he would violate this prohibition. He is compelled to send her away.
Even if she did not become fully married but only “betrothed” to another man she would be forbidden to her first husband.
If she had immoral relations after the divorce she would not be forbidden to the first husband.
This prohibition includes the rule that a woman who is unfaithful to her husband through immorality becomes forbidden to her husband. As it says “after she has been defiled” and here she is defiled.
If she was raped and not the wife of a Kohen, she would not be forbidden to her husband.
This applies in all place at all times.