The standard way of doing this is that your customer provides you with SAML meta data. This is a standard format for sharing information about your SAML setup. This also contains the public key for the private key they use to sign.
SAML metadata can be exported from OpenAM. This can be done by the use of export metadata page. Have your customer go to the following URL
https://<IdP_HOST>/openam/saml2/jsp/exportmetadata.jsp?entityid=IDP_ENTITY_ID
From there the metadata can be exported to XML.