If you know something that he missed, like a right turn signal (which wasn't used here), then a heads up reminder is appropriate. Otherwise, unless you're mentoring an inexperienced rider, no action is called for. Your friend had the same information you did, and theoretically comparable road experience and street sense. He made a decision, and by the time he made it, it was probably too late for your warning.

Advance warning of things like potholes, or cross traffic at an intersection make sense because the forst rider has the news before those following, but from the back warnings of things you are coming up on are comparably to having mother in law in the back seat.