My oldest dd just moved into a new place by herself after spliting with her boyfriend. Her cat is now throwing up and seems really stressed out. Has anyone ever heard of anything like this?
I second the vet visit to rule out any medical causes. Some cats are very sensitive to chemicals, smells, and even sounds, so moving can be tough. Maybe his new home used to be home to a dog or another cat, and he is reacting to that by stressing out. We went through this with one of our cats.
He probably just needs some time to adapt. If that is the case, I've had some good luck with the Feliway diffuser. It contains the pheromone found in cat facial glands (the one that marks stuff as "theirs" when they rub their chin on you or your stuff) and is supposed to calm them down. We've had better results with this for our cat who normally rubs his face on stuff a lot than the one who rarely rubs his face thoguh ... maybe not all cats know what the hormone is supposed to mean.
Just ditto to the above advice. Get to a vet, maybe try the feliway. There's also a pet version of rescue remedy which you could use (assuming there's no medical reason you find for the behavior and she still suspects it's stress).
If it does seem like the separation is hard on the cat, some kind of clothing that smells like the ex-boyfriend that can be kept around for the cat may be helpful. As might extra catnip.