Cats that don’t like eachother

Hello! I am taking care of someone’s cat as they cannot have her with them right now. We have other animals and theres only one room we can keep her in, the problem is, it’s with my other cat. At first they were alright and kept to their separate parts, but now they’ve almost fought twice now, with my cat(she’s larger) going after the other, we can’t split the room up or get rid of one, but i don’t want them to fight as my friends cat doesn’t have her shots. What should i do?