Yes, she followed her heart, and that gave her a lot of heartbreak too. Does that make her foolish? The question now is: Do shrewd people, who weigh the situation around them, make less mistakes than emotional fools? I don't think so. All of us are deceived at some time or the other, not because we are foolish but because we are human. It is not possible for ANYONE in this world to always take right decisions.
People who are always wary of being fooled by others are fooled by themselves. Rigid logic and street-smartness may get one brownie points in the dog-eat-dog world, but eventually it leads to emptiness and loss of mental peace. Isn't THAT foolishness?
Personally, I would rather follow my heart and be fooled a few times, rather than be worldly-wise and be on tenterhooks all the time for fear of being deceived.