The Muslim must fear Allaah and offer the five daily prayers in congregation in the mosque, unless he is excused because of sleeping, sickness and so on.
See question no. 8918.
With regard to Maghrib prayer in Ramadaan in particular, there is a great of neglect concerning it among many worshippers. The imam should be at the head of the worshippers in this prayer for a reason other than the fact that he is obliged to pray in congregation; this reason is fulfilment of the trust to which he has been appointed, or the task with which he has been entrusted.
If this imam neglects to pray Maghrib in congregation in the mosque, this does not mean that you are sinning or that it is permissible for you to pray at home; rather you must offer the prayer in congregation in the mosque, even if the imam is not present. Each person is accountable for his own deeds. Even if he does wrong, you are still obliged to do the right thing and avoid doing wrong, so as to preserve this ritual which is one of the pillars of Islam.
It was narrated that ‘Abd-Allaah ibn Mas’ood (may Allaah be pleased with him) said: Whoever would like to meet Allaah, may He be exalted, tomorrow as a Muslim, let him regularly offer these prayers where the call to prayer is given, for Allaah has prescribed the Sunnahs of guidance to your Prophet (S) and they (the prayers) are among the Sunnahs of guidance. If you pray in your houses like this one who stays away from the mosque prays in his house, you will have forsaken the Sunnah of your Prophet, and if you forsake the Sunnah of your Prophet you will go astray. There is no man who purifies himself and purifies himself well, then he goes to one of these mosques, but Allaah will record one good deed for him for every step he takes, and will raise him in status one degree thereby, and will erase one bad deed thereby. I remember when no one would stay away but a hypocrite whose hypocrisy was known, and a man would come staggering between two others in order to stand in the row. Narrated by Muslim (654)
Imam al-Shaafa’i (may Allaah have mercy on him) said:
There is no concession allowing the one who is able to pray in congregation not to do so unless he has an excuse.
Ibn al-Qayyim (may Allaah have mercy on him) said:
Whoever studies the Sunnah properly will realize that offering the prayers in the mosque is an individual obligation unless there is an excuse which allows an individual to miss Jumu’ah prayer and prayer in congregation.
Then Ibn al-Qayyim said:
What we believe to be the correct view is that it is not permissible for anyone to stay away from praying in congregation in the mosque except one who is excused.
Kitaab al-Salaah (p. 166)
The way of the Prophet (peace and blessings of Allaah be upon him) is the best of ways. He would break his fast with fresh dates; if none were available, with dry dates, and if he could not find any dates, he would break his fast with water, then he would go and pray Maghrib.
It was narrated that Anas ibn Maalik (may Allaah be pleased with him) said: The Prophet (peace and blessings of Allaah be upon him) would break his fast with fresh dates before praying. If there were no fresh dates, he would eat a few dried dates. If there were no dried dates, he would drink a few sips of water. Narrated by al-Tirmidhi (632); classed as saheeh by al-Albaani in Saheeh al-Tirmidhi.
What this imam is doing is contrary to the way of the Prophet (peace and blessings of Allaah be upon him), so you should advise him to come back to the right path.
And Allaah knows best.