A single man commits adultery with a married woman. Later the woman divorces her husband, and wishes to marry the single man with whom she had the meretricious relationship. Is this allowed?
No. See Mishnah Sotah, 5:1:
כְּשֵׁם שֶׁאֲסוּרָה לַבַּעַל, כָּךְ אֲסוּרָה לַבּוֹעֵל, שֶׁנֶּאֱמַר (שם) נִטְמְאָה, וְנִטְמָאָה, דִּבְרֵי רַבִּי עֲקִיבָא. אָמַר רַבִּי יְהוֹשֻׁעַ, כָּךְ הָיָה דוֹרֵשׁ זְכַרְיָה בֶן הַקַּצָּב. רַבִּי אוֹמֵר, שְׁנֵי פְעָמִים הָאֲמוּרִים בַּפָּרָשָׁה אִם נִטְמְאָה נִטְמָאָה, אֶחָד לַבַּעַל וְאֶחָד לַבּוֹעֵל:
Just as [the married woman who secluded and defiled herself with another man] is forbidden to her husband, so is she forbidden to the paramour, as it says, “She was defiled,” “and she was defiled” [i.e. the extra “and” comes to include someone else] - these are the words of R’ Akiva. R’ Yehoshua said, “Thus did Zechariah Ben HaKetzav expound.” Rebbe said, “It says twice in the passage, ‘If she became defiled’ - once for the husband and once for the paramour.”
No, it is not allowed. She may not marry anybody with whom she had sex while married to her husband.