Policies: Routing a subset of another ISP's address block

We have a situation where we have a client who wants to be dual-homed for redundancy. They are not large enough to get addresses from ARIN. Given that they are wanting us to allow another provider to route a subset of one of our address blocks(5 /24's out of a /16). Looking for some recommendation/dangers and general policies in reference to this. Thanks for any input. If this is the incorrect list to post this on please let me know. thks -- David Harrison