Or HaChaim on Genesis 38:14/אור החיים על בראשית ל״ח:י״ד

1 כי ראתה כי גדל שלה, for she saw that Shelah had reached marriageable age. What is the meaning of the word והיא in our verse? Why did it not suffice for the Torah to say ולא נתנה לו לאשה? "and she had not been given as a wife to him" without the additional word והיא? Perhaps the verse wishes to inform us of two things. 1) She saw that Shelah had grown up and it was plain to see that Yehudah had not kept his promise when he asked Tamar to wait until such time as Shelah would grow up. 2) She saw through רוח הקודש, divine inspiration, that she was not destined to become Shelah's wife. This is why she could take up a position at Petach Eynayim and successfully seduce Yehudah. The long term results of Tamar's conduct speak for themselves. She deserved every possible credit.