Quoting Mr. Knowitall
When subleasing is allowed, both the landlord and the remaining tenant may reasonably reject proposed alternative tenants. Also, most potential replacement tenants are likely to quickly lose interest if the tenant who remains in residence seems hostile to their presence.
If it is the roommate's fault that a replacement tenant is not brought into the unit, despite the proposal of at least one suitable replacement tenant, then the roommate who vacates can attempt to use that fact as a defense if sued for a percentage of the rent after moving out. However, the remaining roommate's behavior has no impact on the landlord's right to collect rent, so if the remaining roommate defaults the tenant who vacates can be pursued by the landlord for any money owed.