Right on, Mrs Trellis !!
I work with fairly well-educated people & that crops up frequently; also in the Canadian media. Hope the BBC hasn't succumbed.
The worst I've seen & I so regret not scanning that letter to the editor, for posterity. Not even a [sic]:
"My husband & I's
taxes are too high."
That was from a businesswoman.
A suggestion re "He said to Steven and I".
Your approach sadly didn't get through. Maybe you were too subtle. (-;
Short of a lesson on pronouns, this has worked on occasion.
Ask them how they would phrase it if Steven wasn't there.