If you don’t care about learning a new tech, but do care about overhead, just don’t reinvent the comment wheel and download something that works out of the box.

related point—where do we point him for help installing PEAR if needed

http://pear.php.net/manual/en/installation.php

I would use a Ruby script if I could install it on a host. Use YAML instead of XML, it’s much easier than XML (even more easier than PHP’s XML, even SIMPLExml):
comments = YAML.load('the-file')

To load the file into a Ruby array/hash, and:

To convert it back to YAML. I wouldn’t use Javascript to convert this to HTML, but I would let Ruby do that. Javascript could load the comments with Ajax, or in a frame (or maybe inline in the page).

But if you want to use PHP:

- No forms library, overkill, too much to learn for a beginner
- Markdown, or if that gives your users too much power, PHP’s equivalent of Rails’ simple_format()
- OpenID seems ok
- No MVC MVC is excellent for this (small) script. Or just a file with MVC separated by database function library, controller code on top of php-file, template after the controller code.
- SimpleXML is simpler, and he doesn’t need advanced things (and you could use xpath for that), so I’d use that. Or just vanilla-inline-PHP for XML generation.
- Of course, there are languages like Ruby/Python/Javascript/Perl/Lua that are (IMHO) much more sane than PHP, but because most webhosts have PHP pre-installed, it might be better for a small script.