Will is still standing in Rosamond's living room, staring at the door Dorothea has gone out.
Rosamond sarcastically suggests that he go after Dorothea and explain that he likes her better.
Will explodes at her. She's taken away the one thing in his life that he cared about – Dorothea's good opinion.
After all, it would be considered seriously dishonorable for a man to try to justify himself at the expense of a woman. In other words, Will can't go up to Dorothea and say "hey, it wasn't me, it was all Rosamond."
For the first time in Rosamond's life, she feels something like guilt or shame.
He finally relents, and asks her if he should come back in the evening to see Lydgate.
When Lydgate gets home that evening, he finds Rosamond sobbing in bed.
He assumes that it has something to do with Dorothea's visit and soothes her without asking questions.