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Why would you report it? Don't you want the girl there?
It isn't any business of the apartment office, but you do need to check your lease. Make sure it doesn't have anything in there about someone else staying. I don't think they can enforce anything about someone else staying as long as you are paying your rent and things aren't getting destroyed.
It sounds to me more like you are upset because you could use some help with the rent. IF this is the case, talk to your roomate and tell him you need other arrangements concerning this. If it is something permanent it might do you good to split it between all 3 of you.