I don't understand why he is so upset!

Does he seriously think that his cat is doing all of these things because he misses his sister?? I would say that most cats are separated from their siblings at a very early age! It doesn't lead to depression or bad behavior!

Even in the wild, male cats just don't hang around after they reach a certain age. They go off looking for mates! They don't hang around with Mom, Dad, and Sister and have sit down family dinners!

Maybe your friend has some other issues he needs to deal with?