Gittin 23A

והוא שהיה גדול עומד על גביו.

And that is the case only when there was an adult standing over him. When the adult supervises the writing, and instructs him to write it for her sake, it will be valid.

אמר ליה רב נחמן: אלא מעתה, גוי ⁠– וישראל עומד על גביו, הכי נמי דכשר?! וכי תימא הכי נמי, והתניא: גוי פסול! גוי, לדעתיה דנפשיה עבד.

Rav Naḥman said to him: If that is so, that anyone who is disqualified from writing a bill of divorce may do so with an adult supervising him, then if the one writing is a non-Jew, and a Jew stands over him and instructs him to write it for her sake, would you also say that it is valid? And if you would say that it is also valid, but isn’t it taught in a baraita: A non-Jew is disqualified from writing a bill of divorce in any event? Rav Huna answered: A non-Jew acts based on his own will. Since he is halakhically competent, he will have his own intentions while writing and may not be relied upon to carry out the intentions of the supervisor. In the case of the Mishnah, since those doing the writing are not halakhically competent, they will write according to the instructions of the supervisor.

הדר אמר רב נחמן: לאו מילתא היא דאמרי; דמדקא פסיל ליה לגוי לענין הבאה, מכלל דלענין כתיבה ⁠– כשר.

Rav Naḥman then said: What I said when I raised a challenge from a case involving a non-Jew is not correct, as from the fact that the Mishnah later disqualifies a non-Jew with regard to acting as an agent in the bringing of the bill of divorce, one can learn by inference that he is qualified with regard to writing, where he is not listed among those who are disqualified.