From the tmux FAQ:
I use PuTTY and my tmux window pane separators are all qqqqqqqqq's! 
PuTTY is using a character set translation that doesn't support ACS line
  drawing. With a Unicode font, try setting PuTTY to use a different translation
  on the Window -> Translation configuration page. For example, change UTF-8 to
  ISO-8859-1 or CP437. It may also be necessary to adjust the way PuTTY treats
  line drawing characters in the lower part of the same configuration page.
That being said, I use tmux 1.8 with PuTTY 0.62, "UTF-8 translation", "Unicode line drawing code points" and a remote locale of en_US.utf8 which works perfectly fine.
You probably have PuTTY configured to use Unicode without using a UTF-8 locale on your FreeBSD box, or the other way round (if I switch my remote locale to C without touching my PuTTY settings I get the behaviour that you describe).