It depends on whether the increase was legally valid and properly notified under the tenancy rules applying to the property. If it is valid and the tenant pays only the old amount, the difference may be treated as rent arrears, and the landlord must follow the formal warning and termination procedures rather than evicting the tenant immediately. If the increase appears unlawful, the tenant can challenge it through the RTB dispute resolution process, while continuing to pay the undisputed rent. Keep copies of all notices and payments, and consider getting advice from Threshold or a solicitor.