If he doesn't have a night rating in his Austrian license, obviously you have to check that (together with other possible restrictions) because it's a FAA license based on, then he could go night flying on his own. But ffor his CPL you have to do 5 hours of night dual and that will be enough. You can give him the night solo privileges but you don't have to. (I would though, just to be save)
In order to fly N-Reg in the US he has the FAA license. So you just treat it as a FAA license. The training/type rating he has in his Austrian license doesn't mean anything.
You always have to do a Flight Review (24 months) and in this case the SFAR73. (12 Months)
Don't let the Austrian license put you in jeopardy.
HillerBee CFI