By instinct, I would say what Eli said.

  Or like the site offers here as a jocular option "Raimunda – feia de cara, mas boa de rima." (an eufemism...)

   http://radioemergencial.blogspot.com/2007/07/tipos-de-mulher-feia.html

   Kidding aside, to be more civil and less offensive (yeah, many a women wouldn´t

take that one lightly like this.) you can say "ela não é tão bonita assim, mas é um mulherão", "ela é bonita de corpo" (mas não de cara), "ela não é nenhuma Gisele Bundchen, mas é bonita", "ela não é bonita de cara, mas de corpo é..." and other ways.

  These are some possibilities, I bet many others will appear here.

  I also would be glad that the women would offer her opinions on the subject; you know, they are way more tactiful (provided that they aren´t in women-women competitive mode!)