I would focus on raising a member dispute
Third parties are not permitted.
If you raise a member dispute and submit evidence ( screenshots of messages etc ) THS will suspend her account whilst they investigate. The hosts will not be able to submit a review during this time .
If THS agree that she breached the terms (which she clearly did by having a third party living in the home ) they may remove her from the platform .
You can ask for transcripts from your phone calls with THS which maybe useful for your dispute .
This thread discusses the member dispute process
https://forum.trustedhousesitters.com/t/member-dispute-process