What is the ruling on not praying in congregation if the Imam is the manager with whom a friend had a dispute?

Congregational prayer is an individual obligation (fard ayn) that should not be abandoned except for a valid excuse. A disagreement between the worshipper and the imam is not among the permissible excuses for abandoning it. Therefore, your friend's intentional abandonment of congregational prayer is a sin, and he must repent and be diligent in attending congregational prayers.

It is not permissible for your friend or the manager to abandon each other for more than three nights, based on the Prophet Muhammad's (peace be upon him) saying: "It is not lawful for a Muslim to abandon his brother for more than three nights; they meet, and this one turns away and that one turns away. The best of them is the one who begins with the greeting (salam)." The presence of animosity and mutual hatred corrupts religion and disturbs the heart. Therefore, they both must repent, communicate, and overcome their differences out of obedience to Allah and His Messenger. They should strive to reconcile between themselves in compliance with Allah's saying: "And reconciliation is better," and "There is no good in much of their private conversation, except for whoever enjoins charity or good or reconciliation between people."