Of course it's not acceptable. It's outrageous.
Contact Airbnb and ask them to cancel the booking, as your guest has let someone who is not part of their booking stay in your apartment while they are away.
Tell the person who is in your apartment without your permission that they need to leave. This is a security risk for you and your property. If they want somewhere to stay, they can register on Airbnb themselves and find their own place.
Tell you guest that you will need to ask Airbnb to cancel their booking at they let someone stay in your apartment who is not part of their booking. Keep all messaging on Airbnb.
How did you find out by the way?
By the way you need to tighten up your house rules - and include one to say that only guests who have booked and paid can visit/stay in the apartment.