If everyone's expected to wear a similar style, I'd go with it--it is her right to have uniformity within her bridal party.
However, if the styles are left up to the bridesmaids (you said she only requested that you straighten--not do a certain style), I would speak with her. If you were to just go along with something that irritated your core like that, regardless of whether it was 'her day' or not, it would only continue to eat at you and your friendship. If she is as good enough a friend so as to include you in her bridal party, surely approaching the hair issue respectfully and reaching a compromise would be the preferred approach to the situation.
All in all, she probably doesn't know how big of a deal this is to you. For most non-black curlies, straightening is no-biggie, a style option and no more or less. Our hair, however, typically carries much more weight with us due to its history--I'd be willing to bet that your friend isn't fully aware of the implications of blatantly asking you to drastically alter your hair texture, even if for a day, and would appreciate your telling her of your discomfort rather than wallowing in it...if she's a good friend, that is.