Here's an excellent article on cat introductions.
The important thing to remember is go slowly and don't try to force it. Take your cue from the cats' behavior. There is no time table.....for some cats it may be a few days, or weeks, or months, or maybe never. At least your cat has had a cat friend and has experienced feline companionship, so that's better than if your cat has been alone since a small kitten.
Living Together - Introducing a New Cat
The important thing to remember is go slowly and don't try to force it. Take your cue from the cats' behavior. There is no time table.....for some cats it may be a few days, or weeks, or months, or maybe never. At least your cat has had a cat friend and has experienced feline companionship, so that's better than if your cat has been alone since a small kitten.
Living Together - Introducing a New Cat