וַתְּיַשְּׁנֵ֙הוּ֙ עַל־בִּרְכֶּ֔יהָ וַתִּקְרָ֣א לָאִ֔ישׁ וַתְּגַלַּ֕ח אֶת־שֶׁ֖בַע מַחְלְפ֣וֹת רֹאשׁ֑וֹ וַתָּ֙חֶל֙ לְעַנּוֹת֔וֹ וַיָּ֥סַר כֹּח֖וֹ מֵעָלָֽיו׃
She lulled him to sleep on her lap. Then she called in someone else, and she had him cut off the seven locks of his head; thus she weakened him and made him helpless: his strength slipped away from him.
(The above rendering comes from the RJPS translation, an adaptation of the NJPS translation. Before accounting for this rendering, I will analyze the plain sense of the Hebrew term containing אִישׁ.)
The label אִישׁ is the most efficient way to introduce an additional participant into an existing situation; and it is often used in this way, e.g., Lev 16:21.
Meanwhile, the fact that the plot to cut Samson’s hair has been disclosed to the audience is what accounts for the definite article; the accomplice’s presence in the wings is inferable from that plot, and from the prior episodes of ambush. The referent is “definite in the imagination,” as is fairly common in Hebrew usage.
As for rendering into English, the NJPS ‘a man’ is standard — but not very informative. Nowadays it places undue emphasis on this figure’s gender. The revised rendering indicates that the referent is labeled in terms of his essential involvement, in order for the audience to grasp the altered situation.