If you are going to be building the entire product, and not being paid a salary, you should have something around equal equity with your co-founder. If you are going to be paid, discount the equity split to something more fair based on how much you are being paid vs. what you normally make for your time (how much you are sacrificing by not having a full time job).

They might argue that it is their idea, I argue its mostly in the execution, which you will be a large part of.