I haven't come across an emacs mode that would offer code completion suggestions based on "knowledge" of the API(s) that the user's environment is offering. To a lot of people this is an issue which prevents them from attempting to use Emacs or VIM when working with rich/large/unwieldy (delete as applicable) APIs.
However I am wondering how much of a problem this would present during day-to-day work. I've been using Emacs with C#-mode to crank out quite a lot of C# code. I also tend to run dabbrev-mode or pabbrev-mode, which tends to take care of the more common function and variable names I tend to use. To my eternal shame I have to admit that I tend to have a browser open on the MSDN website to look up the rest - those APIs that I don't use often enough to remember. Another potential helper that your colleague might want to look into is icicles, which may also be a step in the right direction. Neither of these libraries however will offer the full breadth of completion support that something the like Visual Studio IDE will offer. I'd see this as part of the trade-off when using a more efficient editor.
As an aside, if your colleague is working in a team and other members working on the same project are using Visual Studio, MSBuild might offer a better solution for building outside of VS than Nant as MSBuild reads the same solution and project files that VS uses (in fact a lot of the build work in VS2008 is handled by MSBuild). The syntax isn't too far away from Nant and with the community tasks added (which gives you NUnit integration etc) and it'll ensure that everybody is using very similar mechanisms to build the executables.