Where does Cosimo get his first glimpse of the widow that his dog has been visiting and what is she doing when he sees her?

Cosimo catches his first glimpse of the girl when she comes riding from the opposite side of the field chased by two male riders, and zigzags so erratically that they are unable to catch her until at last she disappears through the trees. Cosimo follows her, a blonde and an accomplished rider, to the point that he can finally see her face, and his hopeful suspicions are confirmed.