Actually, your designer shouldn't be continuing to work on your mockups in the first place. After the mockup is complete, it gets turned into a template and then everyone works on that. The mockup was just a proof of concept to the customer. It wasn't meant to keep being used.

Also, be careful with your mockups. If you're demoing functionality, you don't want the looks of the site to be pretty, and if you're demoing the visual appearance, you don't want the site to work at all. If you mix the 2, the customer will get it in their head that most of the work is already done, when it's not even close.