What is the legally prescribed action required from the Imam and congregants when one of them faints during congregational prayer, and what expiations result from their leaving the prayer, and what are the cases in which it is permissible to leave the prayer and how does one return to it?

There is no objection to interrupting the prayer for a legitimate reason, such as the fear of losing valuable property, or aiding someone in distress, or alerting a sleeping or heedless person to danger. The Companions interrupted their prayers to defend Umar ibn al-Khattab, may Allah be pleased with him. Therefore, it is permissible to interrupt the prayer to aid a worshipper who has fainted, and this is limited to those by whom aid can be provided. There is no expiation for one who leaves the prayer with or without an excuse, but whoever leaves without an excuse is sinful and must repent, and the prayer is resumed anew upon returning to it.