What is the ruling on the prayer of one who intended to pray Maghrib, then intended Isha with the Imam who is praying Isha after the salutations of Maghrib?

What the worshipper did is correct, though it would have been better for him to wait until the Imam sat for the Tashahhud, then to make the salutation along with him. It is permissible for a follower whose prayer is completed before the Imam's to separate from him and make the salutation, or to wait for him during the Tashahhud until he makes the salutation along with him. The prayer is not invalidated by separation due to an excuse, such as the inability to follow; this is the view of the Shafi'is. In contrast, the Malikis and Hanafis believe that the prayer of one who separates is invalidated.