You have a couple of options.
As already mentioned above, you can always live trace and go from there.
If you don't have Adobe Illustrator, Inkscape is an opensource alternative.
Those lines look pretty sharp though so I get the impression that you either want to tidy the sketch up a little or make a workable digital version.
To tidy it up a little;
Rather than colouring in all of the lines and sections with the brush tool, you can use either Photoshop or the opensource image editor Gimp to mess with the levels, jack up the brightness and the contrast and then go from there.
You'd end up with something like this as a starting step;
Some more steps are mentioned here about cleaning up hand drawn sketches as well as a wealth of online resources :)
Now if you want to make an illustration of your friends drawing questions like that have been answered before and there are some tutorials online too.
Best of luck, hope I've helped.
Welcome to the site :)