@Jana27 I've had a very lengthy correspondence with Airbnb over this very topic, and the rep from Trust & Safety made it quite clear that they will not mediate in any capacity over issues related to downloading. Money quote:
"While we understand that you're facing fines associated with illegal downloading, we're regrettably unable to offer any compensation with respect to these fines. As per our Terms of Service, we are excluded from liability with respect to loss of data or any related fines or fees, or incidental, special, or exemplary, or consequential losses of this nature"
You do have the opportunity to use your review to make it difficult for the guest to book again, but you'll have to stop short of making a direct accusation of illegal conduct if you want the review to stay up.