וַיֹּ֣אמֶר אֲבִימֶ֔לֶךְ מַה־זֹּ֖את עָשִׂ֣יתָ לָּ֑נוּ כִּ֠מְעַט שָׁכַ֞ב אַחַ֤ד הָעָם֙ אֶת־אִשְׁתֶּ֔ךָ וְהֵבֵאתָ֥ עָלֵ֖ינוּ אָשָֽׁם׃
Da sprach Abimelech: Was hast du uns da angetan? Wie leicht hätte einer aus dem Volke deiner Frau beigewohnt, und du hättest Schuld auf uns geladen!
Rashi on Genesis
אחד העם ONE OF THE PEOPLE — the one singled out from the people — viz., the king (cf. Targum).
Sforno on Genesis
כמעט שכב אחד העם, the person described here as אחד העם, a euphemistic expression, was the king himself, who was after all, אחד העם, in a unique position compared to the common people. He had considered himself above the law doing whatever he pleased; or he had imagined that seeing that he was the king no one would dream of refusing to sleep with him, on the contrary every woman would be pleased to have been chosen for such an experience and that Yitzchok would be highly honoured to have his sister marry the king.
Radak on Genesis
ויאמר.. כמעט שכב, he meant that it had been very likely that someone among his people would have slept with Rivkah seeing that Yitzchok had represented her as being his sister. They would not have considered such a sexual encounter as something sinful and therefore Yitzchok would have been to blame for any guilt arising from such an encounter.