Beating 24 minutes for a beginner is wholly over the top IMHO.
If he’s a beginner to road biking, perhaps, but a cyclist with a bit of experience and skill, on a flat course, cool night and no wind, I’d say 24 is a realistic target.
A fairly experienced roadie will have the skills/knowledge/roadcraft* to chop a minute off the time of a similarly fit mountain biker.
(*When to draft, when to put up a sail, how to stay out of the wind, where to save energy etc)