Answer:
The man in the yellow suit saw and overheard the whole conversation of the Tucks and Winnie.
This scene can be good for the Tucks were tired of living "forever" and this provided a chance for them to be in contact with a "real, living person".
It can also be bad in that he now knows the secret to their longevity and may  try to do something sinister with his discovery.
Explanation:
Natalie Babbitt's Tuck Everlasting revolves around the 'magical' world of the Tucks who had been 'unintentionally' blesses/ cursed with living forever. Their secret of how they had come to this situation was overheard by the man in the yellow suit, who had been looking for the Tucks after hearing about story of his grandmother's friend who had married into a strange family. And all these secrets came to light when Winnie encountered the Tucks in the woods, and tried to drink the water in the woods which prompted them to tell of their condition.
This discovery of the man in the yellow suit about the secrets of the Tucks is good in that it led to the discovery of the real intention of the man. Also, it led to the discovery of new friends for Winnie, who felt a deep connection with the Tuck family. It also gave the Tucks a new chance to experience the presence of "a real, honest-to-goodness, Â natural child!"
But, it may be bad in a way that he will use it in a more sinister manner. He wanted to 'share' the water to "certain people, people who deserve it." This plot led to the death of the man in the hands of Mrs. Tuck who was convicted to be hanged and thus led to the 'escape' of the Tucks from the place. Winnie lost her friends but saved the life of the woman.
