I'm going to tell my roommate I'll rent her room out if she doesn't pay
Why is your roommate not paying her rent?
She doesn't stay over long enough to live here.
It's terrible to rent out someone's room if they are still living in it. You should respect people's property.
She doesn't need that money anyway, I will use it for my own needs.
Give your friend a heads up before doing anything rash. If the property is yours, then they need to pay rent or move their stuff out so you can rent it out. Explain to them how wasteful it is to not be occupied and used.