Hi Kumar,
If Customer does not want to give you access to their SAP:
Option 1. Ask them to provide the IDoc WSDL. You can then use that in your mapping and make the communication as SOAP. For B2B, SOAP is better over ALE communication.
Option 2. Ask them to provide IDoc xml and then you can convert the xml to XSD and use in your mapping.
http://www.****************/Tutorials/ALE/IDOCToXML/IDOCToXML.htm
There are number of tools available for converting XML into XSD.
Regards,
Nabendu.