To be more accurate,
every leader can declare war while friendly with the target civ ( or in any other diplo stance ). What they don't do is
start planning a war above a leader dependent threshold, a quite diferent situation.... besides the fact they can change the diplo stance after starting the war plans ( and this can make a huge diference, especially in maps where the AI can't reach immediately the target via land ), the AI can start wars by a handful of other mechanisms besides this kind of planning ( AP resolution, peaceful vassaling,
refuse of a demand ( this one happens a lot ), events, Defensive pact, bribe .... and even a bug while playing with no espionage option in 3.17 and above related to a event that has only one option allowed in that setting, that is war ), so use this knowledge with a pint or two of salt
P.S the Catherine situation is a little diferent: Catherine will not start planning a war vs a target she is friendly with, but will acept be bribed against it
... a "I'll not fight you in even terms, but will backstab you" situation that happens a lot with the stock leaders of Civ IV