yeah, funnily enough nobody wants to be friends anymore...!
i'm a big fan of the 'put your crap heir in an impossible military situation and 'forget' to support him' option. and if that doesn't work, you generally get a combat demon who can fight anything.
sometimes you get no choice, such as when you have a 60-odd year old king with only one son. he may be an idiot, sire, but he's the only idiot we have...