If you look on Airbnb Help there is an email contact, your client can use to report their place is being illegal sub-let by a tenant. They should also call Airbnb on the relevant customer services number. And follow up on social media if they provide proof and Airbnb doesn't take down.
If I was in that situation, I would also post notices on the property.
Presume your client has also started legal proceedings to evict as this is an ongoing issue? @Daniel6954
@Anonymous Airbnb does actually have a function to shut down illegal listing where the landlord can show this is down contrary to the lease.