Not having an iPhone, will stick with what I know...
Google offers a number of applications / features, one of which is Google Documents (allows the sharing of spreadsheets and other types of document, inc wp docs.)
Had not tried out their Calendar function before, but just signed in and that also has a Tasks facility, which might be ideal for you
How well these would work via iPhone I cannot comment, however...
At least having it on the web would mean no synchronisation problems between yourselves and the work colleague, and you have control over who can access it, so makes it available to logged in users (yourselves, colleague) and yet available worldwide (anywhere you can find a browser, really, from Mac, Linux, and any other operating system offering net access).