Dorothea cleanses Lydgate's tainted loan by replacing it with her own money. Although it doesn't stop Lydgate from leaving Middlemarch, it removes Lydgate's humiliating relationship with Bulstrode. His reputation in Middlemarch is damaged beyond repair; the virtue of Dorothea's act of kindness toward him is that Lydgate knows that at least one person in Middlemarch has a good opinion of him.
At the last, even Bulstrode himself makes a small step towards redemption. Through his actions, Fred and Mary are finally able to marry. Caleb Garth himself is good enough not to lump Harriet Bulstrode in with her husband's crimes. He doesn't entertain himself with her misery like some people do.
Dorothea's final situation illustrates again the regrettable restrictions on access to the public sphere for women. She makes one independent act by helping Lydgate, and her assistance is a much-needed balm on the misery and stress of the Lydgates. However, her marriage to Will signifies her return to the narrow domestic sphere. The promise she shows as a reforming philanthropist is never realized independently. She lives her chosen occupation through her husband. Will becomes the ardent public advocate of reform, and Dorothea lives in his shadow as his wife and the mother of his children. Rosamond and Lydgate never really achieve an easy peace in their marriage, so it is unclear whether Dorothea's help made much of a difference.
In short, the ending is ambiguous. We have followed two unhappy marriages to their conclusion. Lydgate's only escape from his unhappy marriage is an early death. After becoming a widow, Dorothea marries the man she loves. We are never sure if she is satisfied with the domestic sphere. The unhappy marriages have failed due to various personality differences, unrealistic ideals of the respective roles of husbands and wives, and the processes of self-deception that seem to mark all human activity.
There is one possible, happy marriage that doesn't happen, however. Farebrother advised Lydgate to marry a "good, unworldly woman." This is the opposite of Rosamond. Farebrother was recommending a woman who doesn't mind waiting through the years it takes to build a lucrative practice. Moreover, Farebrother was recommending someone who appreciates Lydgate's passion for his vocation. This advice clearly suggests Dorothea; the marriage that doesn't happen is, obviously, the one between Lydgate and Dorothea. She shares his passion for reform and his human concern for the alleviation of suffering. She doesn't care for wealth. She also showed a strong interest in the New Hospital itself. However, they met before each of them had obtained real life experience. They met before they had lost their unrealistic idealism about marriage. They were married to other people before they could appreciate one another.
It is difficult to tell whether Dorothea would have been able to exercise a public role in the hospital had she married Lydgate, but there is some indication that she would have. The best wife for Lydgate would have been a patient, equal, sensible partner. Dorothea would have been that woman. However, the vicissitudes of fate worked against their marriage.