Raymond has a significant change of heart in the midst of the outbreak; he goes from a self-proclaimed "stranger" in town trying to escape to a man who admits that the plague is "everyone’s problem" and risks his own neck to fight against it.
The easy explanation is that Raymond’s character grows, changes, develops throughout the course of The Plague. Through his conversations with Rieux he becomes enlightened, is made aware, has the necessary intellectual inspiration to realize that Rieux is in the same position as he and that Rambert was being a coward. Having seen the light, Rambert changes his mind and becomes a better person while three fairies somewhere get their wings. Yawn.
The more fun explanation is that Rambert doesn’t change at all. He and Rieux simply logically think through the predicament and come to the conclusion that the duty to mankind is stronger than the duty to love.
More interesting yet would be the argument that Rambert and Rieux actually don’t know if they’re making the right decision. Welcome to moral relativism, or at the very least, moral uncertainty. Because there is no objective truth, it is impossible to say which is the more correct course: taking care of their wives or tending to the sick in Oran. The Plague could suggest that which is "right" doesn’t matter terribly; what matters is that a decision is made in full consciousness and action then taken. This would be the more existentialist view. The humanist view, on the other hand, would be that staying in Oran was morally correct decision.
So what can we conclude, concretely, about Rambert? About as much as we can conclude concretely about objective truth in The Plague. But uncertainties aside, Rambert is a big part of the reason that Rieux comes to the conclusion he does regarding man and hope. He cites Rambert’s reunion with his lover as evidence that, if man only hopes for love, man can sometimes be satisfied.
Is Rambert really satisfied? We have to say, we’ve always taken issue with this conclusion of Rieux’s. Rambert anticipates his "wife"’s arrival with more nervousness than joy; having been irreparably changed by the plague, he struggles to shake off his indifference. If the citizens truly do reach the point where they no longer hope for salvation, what happens when salvation arrives? If Rambert gave up on his love, then was his former hope rendered impossible to satisfy? Well, it’s something to think about anyway.