Is it permissible for someone who finds a congregation in the last Tashahhud and the Imam does not notice him, to pray alone after the Imam says the taslim and the worshippers complete their prayers as latecomers?

If someone enters the prayer as a ma'mum (follower) and then discovers that his imam (leader) has finished, he should change his intention to praying individually and complete his prayer, and there is no harm in that. Evidence for the permissibility of changing intention from ma'mum to imam or vice versa, or from an individual worshipper to an imam or vice versa, includes: 1- Ibn Abbas's prayer with the Prophet, peace and blessings be upon him, at night, where the Prophet changed his intention from an individual worshipper to an imam. 2- Abu Bakr's leading the people in prayer, then his stepping back and the Prophet, peace and blessings be upon him, leading them, so the Prophet changed his intention from a ma'mum to an imam, and Abu Bakr changed his intention from an imam to a ma'mum.