So now you know how to proceed. How old is she? Has she foaled before? According to her age and previous foals, she might always have a bit of a belly. If she's younger, then she could still be carrying worms or just needing work and a good diet. No good cutting back feed to condition her, if she's not being worked too.
What is her current diet and work load? Has she been wormed with different/rotated wormers? Have you see her pass any worms?
I wouldn't think too badly of the person who thought she might be pregnant. I might have thought so too.
She's a pretty girl. What do you do with her?