If the imam is delayed for the congregation, and one of them stands up and prays with them, and then the imam arrives, is their prayer valid or not?

The Shaykh : He says: If the imam is delayed for the congregation, and one of them stands up and prays with them, and then after the prayer ends the imam arrives, is their prayer valid, or do they have to repeat it with the imam?
Some scholars hold that their prayer is not valid, but the correct view is that it is valid. The apparent meaning of the saying of the Prophet (peace and blessings be upon him): is that the imam is present. However, this is a case of advancing the prayer. As for if he does not arrive, they are sinful unless he is delayed for a long time and it becomes difficult to return to him. As for the prayer itself, it is valid.