Admin stuff takes quite a lot of time in a small business, in my experience (3 small businesses so far). Also just doing the design implementation stuff isn't enough - you do have to get out and win the work. 50:50 sounds about right IMO.
That's assuming he is doing inital design (graphical prototype, you'll need usability input here) and preparing graphical elements for the coded pages; as well as the up front promotion and sales.
Kinda depends on what sort of sites you're doing. A heavy app with lots of DB design or lots of scripting would need more weighting towards you. A simple graphical "this is us" site the weight is more towards him (and you're probably better outsourcing the coding of that sort of thing anyway).
FWIW