Legal or not legal player

[splitveer]

We had a situation last night at the beginning of the 4th quarter. Both teams came out of the huddles and team A only had 4 players on the floor. Team B was good with 5 players. Team A coach recognizes the mistake and sends a player to the table. The player checks in after the 15 second warning buzzer. I turn and ask the table if he checked in in the time allowed. They said no. In the meantime my partner put the ball in play as he got on the court. How should this be handled and is he legal?

My thoughts are this. Once a player enters the court illegally and play resumes that player now becomes a legal player. Am on the right track? If I was on top of things a little quicker I would of blown my whistle to stop play. Put the player back at the table and brought the proper player in the game from the end of the third quarter.