I have an older version of QB, but I have not seen any upgrade, even with the same version, that did not make the data file incompatible with the copies that were not updated.
I don't have any clear solution, but I can outline your options as I see them at this point:
1) Have your volunteer Board Treasurer upgrade to 2009. As an upgrade it shouldn't be too expensive. You could pay for part of the upgrade or perhaps get someone to donate the amount needed. There may be other ways as well for your person to get their copy upgraded.
2) IF you have back up files with the old version, you could remove QB 2009 and reinstall the old version.
3) If you set up remote access with something like PC Anywhere or logmein.com, your volunteer could access the machine with your QB 2009 and work on it that way. This may or may not be a workable situation due to a number of factors, but it is possible.
4) If your volunteer is only reviewing the data, perhaps you can export a reviewer / accountant's copy that they WOULD be able to use on their PC? I don't know about the specifics for this, but it seemed like it might work.
Christian