2 problems with Cucumber

For OO you can use domain driven design. Cucumber is used for behavior driven development.

It's just up to you to use the right tool for the job.

To succeed in a project is to involve the client. So I would concentrate on that over enforcing a tool he's not comfortable with. It's great if he thinks about the story and the done criteria's. The tool you used to register/followup doesn't really matter.