What is an appropriate ratio of ab work to lower back work?

There is no one ratio or answer for all clients. It is possible for some people to have overactive abdominals and weak low back. It is possible that some clients have tighter lower back muscles. We should all be using functional core training that is restorative and based on each client.

To the OP’s point, however, you can’t just train the front of the core and ignore the back. I exercise the low back via superman, cobra, quadriped (bird dog), pelvic tilts, cat/camel. All are in Chapter 9 of the ACE Personal Trainer Manual, 4th Ed, pp. 254 to 271.