She knows that his conscience is working on him and has made him sick. She realizes that he was appealing to her that night on the scaffolding to protect him from his enemy, from Roger Chillingworth.
And Hester decides that she should help him, despite the fact that he has done literally nothing for her for the past seven years.
Yep, Pearl is now seven. The townspeople have developed a grudging respect for Hester, who's certainly worked hard enough for it: she's been pure inwardly and outwardly even since that little adultery thing.
People are even saying crazy things like, "Hey, maybe the A stands for Able"; or, "Maybe the scarlet letter actually means that she's holy."
Hester knows the truth: the "A" has hardened her against ever feeling passion or affection again. Apparently passion and affection are crucial components of womanhood, because this means that she's no longer a woman.
Sometimes, she even wonders if it's worth being alive. Maybe it would be better to send Pearl to Heaven immediately, and follow herself?
Luckily, she ends up deciding against the murder-suicide—but, the narrator says, the fact that she thought about it at all means that the scarlet letter hasn't done the work it was supposed to do.
But seeing Dimmesdale's oppression actually makes Hester feel bad, so she resolves to help him.
And she gets her chance, when she runs into Chillingworth in an isolated part of the peninsula while she's out walking with Pearl.