Thank you for all of your replies and thoughts. I'm truly torn about what to do at this point. I posted my health update about Em in the other thread - which is basically that we still don't know what's wrong.
I'm going to try to give a detailed cycle of events in the hope that this will be even a little illuminating. Forgive my estimates with times:
1. 3-4 weeks ago: brought home Berry (1 year old female).
2. Cats tolerate each other - Em not super happy but was eating for that first week. He gave Berry a couple of swats but there was nothing extreme going on.
3. 2-ish weeks ago - Em vomits A LOT. We thought it was in response to Berry, who had escaped from safe room and was sitting in one of Em's places in the apartment. He basically looked at her and threw up, but then wouldn't stop throwing up until it had gone from food to clear bile.
4. This happens again the next morning. Berry is not visible.
5. Em and Berry were together, not separated, for a while for the first time within a day or two of this.
5. Em begins to spiral - we begin to notice the lack of eating and lethargy. We separate the cats again completely.
6. Cats are kept separated. Berry does not leave her safe room (the bathroom). Em goes to ER.
7. Goes home, no improvement. A friend suggests removing Berry from the equation to see if it helps. We do. She is at a friend's.
8. Berry has now been out of our apartment for almost 2 weeks. In the course of this, he has still been back to the vet several times. We purchased Feliway and plugged it into the bathroom to see if that would help.
9. This week Em got a new nausea med. He seems perkier and less depressed/lethargic, but is still barely eating. He has, at this point, lost over a pound (gone from 13-11.75).
Also, some history about Em before we took him:
He was living with another family as an indoor/outdoor cat. They adopted a dog and apparently they were fighting so much that one of the animals had to go, so we took him in. He's been the only cat in our place for almost a year, but he frequently seeks out the "hallway cats" for play and interaction. In fact - he sought them out last night!!! So.....meh?
At this point I am just confused. I have to decide what happens with Berry by Wednesday, because my friends can't keep her indefinitely. I would like to bring her back to try again, but I also don't want to put Em at risk. I'm so angry that this situation has even happened.
Also I should clarify - my bf isn't guilting me about the cat - I sort of feel guilty on my own (and also Em's foster person, who has been following this story, seems to think it is my fault as well). He and Em have an extremely strong bond, and I can tell he is really scared about Em, which is why he is so adamant about not trying again with Berry. Part of me feels he is right, but the other part, who wants to keep Berry and is unsure she is the cause of this, doesn't.
Sorry for my long essays. This is such a crazy situation and it's really hard to differentiate Em's health issues from his potential issues with another cat in our apartment.
That said, thank you all so much for your advice and support. <3