They want you to do SEO on a website that isn't finished? When you say the front end is complete but they're still working on the back end, what does that mean? Is the website functional? Is it "open to the public"? Does it have content available you can work with? what is their time frame for completing the back end work?
If the site is open to the public and has visible online content you can work to promote, there's no reason why you couldn't get started with an SEO project. But you have to have some high quality content to work with. If all they display to the public is an "under construction" page, or if the content available is minimal until they get the back end finished, you might do better to say "no" to this project.
If all you've got to work with that's publicly available is an "under construction" page or minimal "coming soon" type content while they work to complete the back end, there's nothing for you to work with for SEO. You aren't going to be able to acquire many high quality links pointing to a website that has no content, and you won't have much to promote through other channels. It would be a waste of your time and their money. If this is the situation, tell them to come back when their site is ready to launch.