The ABC of Harp Playing by Lawrence and Salzedo will help you with adapting piano music. Not all music can be adapted. It takes a lot of time and skill. He’s not being very fair, and must not know much about the harp. There are plenty of orchestra parts for harp. Good pieces to start on are by Russian composers like Rimsky-Korsakov, Borodin, Tchaikovsky. I published a list of harp parts arranged by difficulty in the AHS Journal Teacher’s Forum a few years ago. It would be good to give a copy to the conductor.