My advice would be to let them sort it out for themselves. Has your cat got a place he can go and hide? That's all that will be necessary really. I doubt if they will fight. There will be lots of yowling and hissing and spitting and eventually they will either be friends or decide to live together and tolerate each other. They will both need somewhere to escape to. Just keep an eye on them. They both have their owners to turn to for love and affection and, despite what people think, cats do relate to people and get security from their owners.
The worst thing you can do is confine them to separate rooms, although it might be necessary for a few nights. I presume they are both neutered!! ;o)