You should stop coding right now. I'm very serious.
If this site is to receive anything more than very simple "design love" such as colors or font sizes, any designer worth hiring is going to want to adjust not just the design, but first and foremost the usability. Great (or even good) user experiences is about more than matching colors and pretty pixels, it's about the complete experience a user has when using the site.
Let's say you code the complete site without any design in place, when the designer comes in and says "there should be no login required for this feature" or "why should use a wiki-like approach instead" or "yeah, this absolutely needs to function offline" or "here we place the facet search-box", you will likely have to start over with some features, possibly even have to start all over if the ideas from the designer completely changes the ideas you had when coding.
If, on the other hand, the designer is invited very early on (before coding has begun) you can all collaborate on the type of experience you should provide and you will be able to craft a solution that is in line with the expected end result. You will be happier, the designer will be happier, the client (your friend) will be happier and most importantly: the user will be happier!
So, to answer the actual question: if you are the programmer and someone else is the designer, you should not do any web-design (this sounds harsher than I really wish, but if these roles are truly split into two people, the programmer should program as the designer designed).
Implementing the requested functionality (in a professional manner) should be your main concern, and it should not be done before you know what is requested (which is unknown before the system/site is designed).
Now stop coding and call that designer for a meeting!