I suggest that you inform member services that the sit has been curtailed and explain why so that the sit end date is changed .
You may also want to raise a member dispute about the sitter having a third party to stay without your permission. Providing the camera evidence and messages . This is to protect yourself from any complaints from the sitter about the sit being curtailed. THS will investigate and potentially remove this sitter from the platform .
Information about member dispute process has been discussed here :
https://forum.trustedhousesitters.com/t/member-dispute-process