"The GIMP has some support for vector drawing in the Gfig plug-in, and the path tool , but these do not provide a complete editing environment and are not well suited to the creation of complex vector diagrams."
But the GIMP has a steepish learning curve (what capable drawing program doesn't).
If he wants a free vector based program then Inkscape -
http://www.inkscape.org/ - would do nicely.
"Inkscape is an Open Source vector graphics editor, with capabilities similar to Illustrator, Freehand, CorelDraw, or Xara X using the W3C standard Scalable Vector Graphics (SVG) file format. Supported SVG features include shapes, paths, text, markers, clones, alpha blending, transforms, gradients, patterns, and grouping. Inkscape also supports Creative Commons meta-data, node editing, layers, complex path operations, bitmap tracing, text-on-path, flowed text, direct XML editing, and more. It imports formats such as JPEG, PNG, TIFF, and others and exports PNG as well as multiple vector-based formats."
But it sounds as though he's do well with something simpler (though Inkscape is quite manageable).
