Your client is absolutely right @Julia-And-Peter0
She should set up her own listing on her own profile and then add you as a co-host. (probably worth you reading up on how co-hosting works on Airbnb Help).
It would not be fair for you to set up her listing on your profile as that would mean you rather than the owner would get the reviews and ratings and if she wanted to then manage the listing herself or choose another co-host, she would need to start again with a new listing.
There is no advantage to the host for you to set up her listing on your profile (she will get a boost on Airbnb as a new listing when she starts).