Is what one of the brothers did, asking the supervisor to change Surat Al-Ma'idah, which contains doctrinal confrontations with Christians, while the Quran was being played in the workplace where there was a Christian colleague, a correct action, or a mistake that necessitates repentance and seeking forgiveness?

Allah Almighty commanded His Prophet to invite disbelievers to Islam. It was the Prophet's practice to recite the Quran to them, which proves the Quran's effect on guiding hearts, as Allah Almighty said: {So remind with the Quran whoever fears My threat} [Qaf:45]. Due to the intense influence of the Quran on hearts, the disbelievers used to advise each other to flee from hearing it and to cause a disturbance during its recitation, as Allah Almighty said: {And those who disbelieved said, "Do not listen to this Quran and speak idly during its recitation that perhaps you will overcome."} [Fussilat:26]. Perhaps the Christian's listening to Surat Al-Ma'idah will benefit him and guide him to faith. It seems your colleague at work intended good by preventing harm to the rest of the employees, so he requested a change in the surah.