1. Create a static route for the NATed network on the customer router.
2. Enable EIGRP on the serial link. (IF the serial and lan of the customer are from the same subnet, then EIGRP will get enabled on both serial and lan networks. To prevent the non-NATed lan address, from being distributed through EIGRP, use a distribute-list preventing the lan network, from being advertised)
3. Redistribute the static route under EIGRP using "redistribute static " command.
Hope this helps!
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus