Is it permissible to pray with someone who has stopped praying in congregation after Fajr or Asr, intending to earn reward for him?

There is no harm in re-establishing a congregational prayer during prohibited times, as it is a prayer with a specific reason. Its permissibility is indicated by the Prophet's (peace be upon him) action of performing it, and by what Ahmad, Abu Dawud, and Al-Tirmidhi narrated from Abu Sa'id (may Allah be pleased with him): "Is there not a man who would give charity to this one by praying with him?" And Sheikh Ibn 'Uthaymin (may Allah have mercy on him) stated that the generality of the prohibition is not intended, and certain individuals are excluded from it, such as re-establishing a congregation.