What did he assume that she heard?
Vilna Gaon: They are suggesting that we will marry. Therefore, I request that you not go to another field, even of mine 1 . Stay in this field!
Malbim: That they judge you favorably here. Therefore, do not go to another field, lest they not be pleased with you.
Malbim: Perhaps the youths or overseer there will hate you.
What do we learn from "v'Cho Tidbakun Im Na'arosai"?
Torah Temimah citing Shabbos 113b: Did Bo'az normally cling to girls 1 ?! Rather, because "va'Tishak Orpah la'Chamosah v'Rus Davkah Bah" (1:14), he realized that it is permitted to cling to her. (Iyun Yakov - if Rus would be forbidden to Bnei Yisrael, Na'ami would not have agreed to convert her 2 .)
Vilna Gaon: Even though I heard that you are proper, it is improper for you to cling to my male youths, only with my female youths. He had females working in the field.
Malbim: Males worked on one side of the field, and females on the other. He told her to work only on the latter side.
Maharsha (113b): He told her to cling to his girls! Even so, since 'Divuk' is usually between man and wife, we expound the word. Bach's text - did Bo'az normally speak with girls?!
If the Torah permits accepting Mo'avi converts, just forbids them to marry Yisraelim, why should Na'ami refuse to convert her? (PF)