Herman-
You'll want to download your own copy of DNN and install it locally (preferably the same DNN version that your friend is running).  If the module you're writing is very content-dependant (relies heavily on what's already on your friend's site) then you can bring down a copy of the site to work with -- see the "backing up a DNN installation" link below.  Note that you will not re-upload the local copy of the site but rather you'll package and deploy the module that contains your custom code.

Check this post on installing DNN locally:  
   http://www.mitchelsellers.com/blogs/articletype/articleview/articleid/250/installing-dotnetnuke-490-locally-nonvista.aspx

If you're new to DNN you'll want to read this one of HOST settings:
   http://www.mitchelsellers.com/blogs/articletype/articleview/articleid/189/dotnetnuke-host-settings-explained.aspx

Here's the bare minimum for getting the framework of a custom module going:
   http://dnnsecrets.wordpress.com/2008/01/19/create-a-helloworld-dotnetnuke-module-in-6-easy-steps/

Additional "how-to" module creation links are here:
   http://www.dnncreative.com/Resources/DNNModuleDeveloperResources/tabid/125/Default.aspx

The HOST->MODULES menu in DNN (must be logged in with a HOST account to see the menu option) provides the means for uploading and installing the finished module.

Finally, check this post about backing up a DNN installation before making changes (like adding custom modules):
   http://www.mitchelsellers.com/blogs/articletype/articleview/articleid/164/how-to-backup-a-dnn-installation.aspx

 
-mamlin