Praise be to Allah.
And upon you be peace and the mercy of Allaah and His blessings.
This depends on what has happened to this worshipper. If waiting until you have finished the prayer will cause him harm, then you should stop your prayer and help him. If waiting until you finish will not cause him harm then you can help him after you finish the prayer. And Allaah knows best.
Fatwa of Shaykh ‘Abd-Allaah al-Ghadyaan.
In such cases you should act according to what you believe is most likely to be the case, with regard to whether waiting until the prayer is finished will cause harm or not. And Allaah knows best.