Severe Cat Aggression?

Sounds like Yogi is stressed. He has a kitty version of PT-SD. Unfortunately sometimes the only solution to this kind of aggression is separation and it sounds like Winnie's owners are unwilling. Most likely it's a case of too many animals in one place. Cat's don't adjust as well as some animals to new surroundings, and especially new cats. It may make Yogi and your other animals upset in the short term to be shut in a room, but it may keep them safe until the attack cat moves out.