@Rebecca300 Tier 3 exception (one of many) to the no hosting rule is
to provide accommodation for any person who was staying in that accommodation immediately before these Regulations came into force or (if later) immediately before the time when the area in which the accommodation is located became part of the Tier 3 area,
so I believe you can let the guest finish their stay.
Google tier 3 legislation and you will find the whole text.