Deadbeat Tenant

Friend of mine sublet her apartment to someone who isn’t paying her. They paid the first month and since then have had made continuous promises to pay next week for the last 3 months but haven’t made any payments. Total amount due is 3-4k.

The sublet was not approved by the Apartment complex.

What would happen if she took them to small claims court? Could this blow back on her from the apartment complex?

Additional info:

She wrote up a basic agreement that was not notarized but was signed by the tenants that they would cover rent and utilities for the apartment at that location/ apartment complex for the remainder of the lease. She has all the texts of them promising to pay (by Friday, next paycheck, etc.).