Or HaChaim on Genesis 38:24/אור החיים על בראשית ל״ח:כ״ד

1 הוציאוה והשרף, "take her out and let her be burned!" The reason for this is that it is the prescribed penalty for adultery amongst the Gentiles as we know from Avodah Zarah 36. It is clear from there that even if the sin did not involve a sexual union forbidden by Biblical law either to Israelites or to Gentiles, the tribunal headed by Noach's son Shem appears to have legislated that a woman who was destined for her brother-in-law was liable to death if she had relations with someone else. As soon as Yehudah realised that he had been her partner, he knew she was free from that decree as it applied only to Gentiles amongst Gentiles, not to partners in a levirate union. Since a father-in-law was not culpable for sleeping with his Noachide daughter-in-law, no crime had been committed seeing Tamar was Jewish. Sotah 10 understands the words ולא יסף עוד לדעתה to mean that Yehudah did not discontinue being intimate with Tamar. This proves that their relationship had not been sinful, i.e. that the father of the deceased could legally perform the rite of the levirate marriage with his daughter-in-law.