If you have Adobe Flash and can edit the files for a
FlashHelp skin of your choice to use Calibri, then FlashHelp would
display the Calibri font on every user's computer.
With most anything on the Web, fonts must be installed on the
user's machine in order for them to appear as specified--otherwise,
they'll default to something else. Flash is an exception to this
because the fonts become embedded (or can be embedded) in the
movie.
The quickest fix is to choose a font for the style sheet that
you expect to find on every user's computer (Verdana is a good
bet). For the WebHelp TOC pane, the font is specified in the Skin
Editor rather than a style sheet.
Hope this helps some,
Ben