T for being out of bounds.

[ChuckElias]

Welcome to the forum! Glad you've found us. As to your question, by rule you were correct. Being OOB for an unauthorized reason, or delaying your return after being OOB for a legitimate reason are both technical fouls.

If the player OOB had no impact on the play in question, I might have first urged him to get inbounds and then explained to him that he's not allowed to do that, as soon as I got the chance.

But by rule, and especially if he then stepped inbounds to receive a wide open pass for a lay-up, you were correct.