I wouldn't have said anything either. I think that "natural" is a subjective term for a lot of people (though I have a clear idea of what it means for me), and I wouldn't quibble with someone's pride over her "natural" birth even if I wouldn't describe it that way.
HOWEVER, when someone is describing MY birth, I feel free to correct them. I get REALLY annoyed when my DH describes DD's birth to other people as a "natural birth," because it most assuredly was NOT.
One of our big arguments when I was pregnant went like this:
ME: I'm not giving birth in the hospital, because I want a natural childbirth.
DH: You can have a natural childbirth in the hospital! Didn't you hear the nice L&D nurse, she said you can turn the lights down and have music and everything!
ME: But giving birth in the hospital just isn't natural to me. And besides, they always try to intervene SOMEHOW.
DH: Well, they won't intervene in OUR birth.
ME: I'm NOT GOING TO THE HOSPITAL to have this baby.
Well, we both ended up eating our words. My HB ended in a (non-emergency) transfer (for exhaustion) after three days of labor... and it was really good, as transfers go, but I ended up getting an epidural to help me rest. And with the epidural came everything else: catheter, IV, electronic fetal monitor, blood pressure cuff, lithotomy position for pushing, etc. Thank heaven I managed to have a vaginal birth, but it was IN NO WAY NATURAL. I have to correct DH on this point constantly. Now it goes something like this:
ME: I am not going to the hospital to have our next baby because I really want a natural childbirth sometime in my life.
DH: But you HAD a natural childbirth in the hospital! You can certainly do it again!