Is it okay if I tell an old friend that I think their new friends are arseholes?
why do you wanna tell your old friend that his new friends are arseholes ?
Because I really believe they are.
It's alright if you don't like your friend's new friends. You don't have to like them.
I want to insult them to make my friend realize that his new friends are useless.
That might have the opposite effect if you are trying to show your friend how rude they are. Tell your friend that you don't appreciate their new friends without resorting to ill mannerisms, they'll value your concern for looking out for them.