A bit too soon !
From an instructional point of view just as I won't let a student solo during daytime without having first done enough navigation training to get to a nearby airfield should the runway become blocked, I would also apply the same criteria to night flying.
With the very different challenges of night navigation to that of daytime I think that it is very unlikely that it would be safe to send a student solo at night until the final stages of the PPL training and even then I would advise caution.