ASN and Peering Problem

We currently have two /19 that we advertise on a single ASN. A client would like to obtain /23 or /22 from us. This is not a problem, except that their primary internet provider is someone else, other than us. I think that they would need to have their own ASN to advertise their portion of our ip space to their peers. My question is, should we provide the ASN or should they apply for an ASN? What is the minimum block considered routable, is it reasaonable to advertise a /23 on its own ASN? Are there any other solutions I haven't thought of? Thanks, Adi