NI on air bnb income

Hi,

Customer has rental properties, which are managed by a letting agent.

He has bought another property which he rents out purely on airbnb - this is not in his residence. He does cleaning, bedding etc in it.

Am i right in thinking that the airbnb income would consitute a trade and be subject to NI as he is going above and beyond the realms of being a landlord - so self-employment? Or would it just go on the rental income pages?

thanks