Email address sharing

A client for a non profit running group has asked me to share the entire data base with a church group to help them promote an up coming event. I have expressed that this is unethical as our members have not given us permission to share their contact information. My client does not see the harm in sharing. I have offered to add the event to our next email promotion. My client feels it will be easier to just hand over the list. Please send any advice or articles you know of to help me explain why we should not do this.

thanks!