Is Someone's a Word?

Someone's can mean someone is or be the possessive form of someone. You can typically figure it out given the context. In your context it means someone is and it should be clear to most, if not all, native speakers. It is a standard form and entirely grammatical.

What is the meaning of someone's?

idiom. : as someone usually or often does. See the full definition.

What is the difference between someone's and someones?

There is no difference in meaning between someone and somebody, but somebody is more common in spoken English, and someone is more common in written English.