Please describe the topology in a little more detail. I also need to know which protocol the customer is using to access the mainframe (SNA, IP, etc.) If they are using IP (tn3270?) then are they able to ping the mainframe (osa adapter?) ip address? If they are using SNA, then you will need to configure bridging on the router (transparent, source routing, etc.) and DLSW on the frame relay connection.
Tim