Is contacting former customers considered a mistake and contrary to Islamic law, and what is the correct approach with a friend who turned against the questioner after he left work?

Exaggeration in love and hatred is blameworthy in Sharia, for the Messenger of Allah, peace and blessings be upon him, said: "Love your beloved moderately, for he may become your enemy one day. And hate your enemy moderately, for he may become your beloved one day." What is forbidden in Sharia is to sell over another Muslim's sale or to buy over another Muslim's purchase, and not merely informing customers about your new shop. It is not permissible to boycott a Muslim for more than three nights, and it must be clarified that your action does not involve any Sharia violation or intention to harm. It is advised to repel evil with good and to beware of the whispers of Satan.