Speaking personally, it would depend a lot on how much handholding the client needs, how much I think my partner would deflect that, and how thin the margins are.
I ran a small freelance consulting business for a few years (put myself through college with it), and had situations where I was the sole interface with the client, and other situations where someone else dealt solely with the client. It depended a lot on the situation, but there were definitely situations where earned significantly more per hour worked by not having to deal with the client, even though someone else was taking a significant cut, simply because I spent more time writing code, and less time figuring out what the heck they wanted.
The best situation I had by far was working with a design house who contracted me to do the backend programming for some movie promotion websites. They handled all interactions with the client, and gave me very clearcut definitions of what they needed me to do, and were able to answer all of my technical questions clearly and immediately.
If your friend can offer you a situation like this, you're likely to come out ahead. However, I suspect that given that you're doing design work as well as programming, you're still going to have to do a lot of interfacing with the client, as you iterate on the design. My suspicion is that you're going to have a lot more interaction with the client than you think, either directly or indirectly.