If this is a single rental property and lease, with the roommates jointly leasing the property, the effect of the lease is normally that the tenants are jointly and severally liable for the full rent. The roommate can verify the terms of the lease by reading that document.

If they are separately renting rooms in the premises under separate leases, with shared common areas, then the nonpayment by one tenant would not affect the other's tenancy -- but that would not be a context in which the tenants would be splitting the rent, as they would each have their own separate rent obligations.

The tenant should discuss her options with her roommate and the landlord, including the possibility that the problematic roommate might vacate with the tenant takin over the entire rent obligation or searching for a replacement roommate, or their buying out the lease.