It seems to me that the Captain could have done a better job of keeping the F/O in the loop.
What part about "CB straight ahead, ask for a 30nm diversion east of track" is not keeping the F/O in the loop? ANY pilot with a couple of ounces of brain cells would realise from that statement that the captain wanted to turn the aircraft 30nm east of track to avoid the CB.
Now, if the F/O thought that was excessive he could have called for it from ATC and then had a quick question of the captain to assertain his thought process. If after having a discussion he still thought it excessive he could log it away for future reference. Being extra safe costs HIM nothing, the company a little extra fuel burn and the flight a few extra seconds. When (if) he moves to the left seat he will get a whole new perspective on his comfort/ safety margins, believe me.
Perhaps the F/O was not keeping the captain in HIS loop?
PP