In all fairness, I would imagine the vet is basing this on the fact that she had a bloody-show, and her udders are bigger -- which would suggest that she is nearing her foaling time. To the vet, it might indicate that she would be expected to foal within a couple of days. BUT if she doesn't, then I'm sure s/he's considering that both mares may have a similar infection -- in which case putting her on the antibiotics which seem to be helping Saffire would be a normal step.
I certainly don't KNOW what s/he's thinking, but just throwing it out.
Is this the new vet or the first one that was so negative?