As a guest, I have done the exact same thing as your guest.
For a wedding, I did rent a townhouse only to realize it would be best for my brother's familiy.
Fortunately, the host allowed me to keep the reservation as is.
I was grateful, as family weddings tend to be a "fluid" thing. A long lost uncle will come. A niece will have an emergency...
I wish there were an easier way with Airbnb to accomplish this.
The "official" way to transfer would be to ask the original reservation holder to cancel and incur fees , releasing new person to book.
Even if you promise that you will rebate the original fees, it is a difficult sell to the people. It is very clunky, time consuming and risky.