Did your radar fail during flight? You should never fly with radar inop if bad weather is forecasted on your route.
This is capt decision,suported by MEL also.I've been always told that the good captain is not the one that comes out of trouble,it's the one that does not enter it.
So,my opinion,at night,radar inop,weather forecast bad -no take off.
Regarding passing bellow CB-you should never pass bellow a mature CB.You can be affected by extreme WS,hail,strong downdrafts.Sometimes,the base of the cloud can be high,m
ore than 6-7000'.This can give you confidence to pass bellow clouds,having good ground clearance.
This must also be avoided,that's what the book says.You can also experience hail,severe turbulance bellow a mature CB,even with high base.
You should watch your a
,so don't leave with radar u/s.
If the captain wish to,tell him your oppinion,study also the met info,maybe he knows something you don't.Then ,of course,it's his decision,based on his experience,which generally it is bigger than yours. Do not be afraid to fly with him,be there to help him if he makes a mistake,no one has a death wish.
Just curious,how the flight ended that night?
Brgds Alex