Before revision: "Somebody else's problem" or "someone else's problem" is used by a person who considers somebody else is responsible for a problem, or for a factor that is "out of scope" in a particular context.
After revision: "Somebody else's problem" or "someone else's problem" is an issue which is dismissed by a person on the grounds that they consider somebody else to be responsible for it, or that it is "out of scope" in a particular context.